But a former senior FBI agent told ABC News that U.S. government negotiators missed the likely final opportunity to free the last American in captivity for ransom almost two years ago, which ISIS said was "still a possibility" in its last email to her parents before her death.
Numerous Obama administration officials were perceived by some families, including the Muellers, as threatening them with prosecution for "material support to terrorism" if they paid ISIS a ransom, so none attempted to. All of the other American hostages were eventually killed by ISIS. The U.S. confirmed Mueller's death in ISIS hands in 2015. ISIS claimed that she died in a Jordanian airstrike, but the exact cause remains a mystery.
"I think they put a lot of faith in the United States government helping them, and there were some people from the government that tried very hard to help them and did their absolute best. And there were some that just didn't know any better, didn't know what they were doing, and so instead of moving the ball forward, they threatened them," Voss said in an interview.
On Sept. 19, 2014, ISIS sent the Muellers an email which reassured them their daughter had not been executed on her 26th birthday, Aug. 14, as the hostage-takers had threatened would happen after a 30-day countdown to pay.
"Kayla's safe release Is [sic] still a possibility considering our demands are met!" ISIS told the Muellers. "These demands are very straight forward and could have easily been achievable a long time ago had it not been for the stubbornness of your government!"
But U.S. airstrikes against ISIS targets, which had begun Aug. 8, 2014, in northern Iraq, expanded dramatically on Sept. 22 in a blitz across Syria. The hostage-takers never replied to the Muellers' emailed pleas again until she was publicly reported dead by her captors the following year.
Asked last week why the U.S. expanded airstrikes in 2014 with almost a dozen western hostages still in the hands of ISIS, a senior administration official told ABC News, "The U.S. didn’t have any great choices here. No matter what we did, there would be a price to pay.”
The final message from ISIS to the hostage's family was among the nine emails the Muellers received from the terrorists -- more than any other hostage's family -- and which they provided to ABC News. The Muellers' 18 emails were primarily written by FBI hostage negotiators. A selection of the emails is posted here.
The Muellers told ABC News that their FBI hostage negotiation team promised to help them make a genuine ransom offer for their daughter's life with messages sent from the family email account used to communicate with the hostage-takers, in Carl Mueller's name. But the emails sent to ISIS obtained by ABC News contain no offer in exchange for her release.
“Carl would say we need to make an offer, and then the [FBI-authored] email would not have anything about an offer in it,” Marsha Mueller told ABC News.
“We were like sheep. We were following what the government told us to do. We had no idea,” Kayla’s father Carl Mueller said in the couple's interview for ABC News' "20/20” segment, “The Girl Left Behind.”
Some counterterrorism officials have told ABC News that the FBI has quietly helped American families negotiate and facilitate the payment of ransom to other terrorist groups since 9/11 in order to recover loved ones held hostage.
Not making a ransom offer to ISIS -- which would have been, in fact, legally allowable under a Bush-era presidential directive -- was the biggest missed opportunity to free Kayla, said Voss, the retired FBI agent.
“There is an exception. And it's when you have the possibility, a reasonable outcome of retrieving the ransom. And of bringing the terrorists to justice. And it's been done in the past,” Voss said. “As far as I know they [Kayla's parents] were never allowed to... They were told if they made any sort of an offer they’d be prosecuted, which is unconscionable.”
Citing classification and privacy concerns, FBI and Hostage Recovery Fusion Cell officials declined to comment about the Kayla Mueller hostage negotiation or about the White House order containing the legal exception which allowed payments as part of a lure with a reasonable chance of denying the hostage-takers the benefits of ransom, National Security Presidential Directive-12.
But a former senior White House official involved in the hostage crisis in 2014 told ABC News there was no way to justify paying ransom to a group like ISIS operating in lawless Syria, where money could not easily be traced.
"The National Security Council and FBI, Department of Justice and Department of Defense carefully considered options to capture the hostage-takers in conjunction with an exchange but none of the options were judged viable," retired Army Col. Mark Mitchell told ABC News last week.
A Green Beret who earned the nation's second-highest award for valor, the Distinguished Service Cross, in Afghanistan, Mitchell has acknowledged publicly that as counterterrorism director in the Obama NSC, hostages' family members such as journalist James Foley's parents have accused him of threatening that anyone who paid ransom would land in a federal prison.
Mitchell denied doing anything more than stating U.S. law to the families but his past comments in those White House meetings have been privately disavowed as inappropriate by some senior administration officials since 2014. The Muellers say Mitchell was one of many Obama administration officials who warned that their donors could be prosecuted and they do not fault him for their inability to raise a ransom fund.
It was almost a year after Kayla was kidnapped on Aug. 4, 2013 before Carl and Marsha Mueller were even able to learn the identity of the hostage-takers as ISIS, much less start negotiating with them.
Though a 10-second proof-of-life video of Kayla by then-unknown hostage-takers was received by her family a few weeks after her abduction in Aleppo, Syria, after leaving a Doctors Without Borders hospital in a marked vehicle from the medical aid group, her parents weren’t able to begin negotiations for their daughter’s release until May 23, 2014. That was the date Doctors Without Borders turned over to Carl and Marsha Mueller an ISIS email address brought out of Syria seven weeks earlier by three of their aid workers who had been held hostage with the young American. ISIS had ordered the women to memorize the email address and to use it to negotiate for Kayla.
But ISIS reached out to the Muellers first -- only hours after the family received the contact information from Doctors Without Borders.
“This message is to inform you that we have the American citizen, Kayla Jean Mueller PRISONER. We don’t want to harm her. She’s like a guest with us at the moment,” the email read. The Muellers were told they could ask three questions only their daughter could answer, to prove she was still alive and in their custody, which ISIS quickly provided:
How did you get the stitches in your eyebrow as a child? Answer: Her older brother Eric was pulling her in a wagon and it tipped over and she fell on her head onto a small rock.
What did you teach your niece to say? Music is______? Answer: Music is Everywhere
What is your friend Moe’s real name? Answer: Her name is Monica and she is a kinder garden friend.
Kayla was still alive, her elated parents concluded.
The kidnappers demanded the release of convicted terrorist operative Aafia Siddiqui or 5 million euros. They said there should be “NO MEDIA EXPOSURE WHAT SO EVER!” Anything less and they’d put a bullet in Kayla’s head, they wrote.
But Voss says the FBI negotiators missed an important opportunity to make Kayla safer from the start, by failing to exploit the cultural importance of guests in Muslim countries.
“This is something huge that they missed, because they should have responded with, ‘No, she's not like a guest. She is a guest and she is your responsibility as a guest',” he said.
The kidnappers also sent an audio of her voice.
”Mom and Dad, I still am remaining healthy. You should have already received the three answers to the proof life questions you provided,” Kayla said, still sounding strong.
The negotiation drama for Kayla played out over the next four months, from May to September 2014, amid an astounding chain of events with potential impact on all the western captives, who included nine journalists and aid workers.
Mosul fell in June and ISIS declared an Islamic caliphate, Army Sgt. Bowe Bergdahl was freed in a Qatar-arranged swap for five Taliban detainees, a Special Forces operation in July to free the western hostages came up empty near Raqqa, Syria, the U.S. began bombing ISIS across Syria and Iraq, and beginning Aug. 19 James Foley, Steven Sotloff and a British hostage were beheaded on video by an English-speaking executioner in retaliation for ISIS being bombed by coalition warplanes.
Throughout this turmoil, however, the Muellers received nine emails from ISIS and in reply they sent 18 email pleas for their daughter’s life composed mostly by a team of FBI hostage negotiators with access to the family’s email account. No other American hostage’s family is known to have received so many emails from ISIS.
But the FBI repeatedly failed to leverage the discussion in a way to get Kayla home, said Voss, who is respected by many government insiders for his success as a senior agent formerly in charge of operations such as hostage recovery for the FBI in Iraq.
“I know that there are some very talented hostage negotiators in the FBI that knew what they were doing. I see no evidence of their voices in these emails,” he said.
The communications quickly deteriorated as the FBI wrote lengthy email after email begging for time to raise a ransom and with the father complaining about money troubles that only seemed to increasingly anger the hostage-takers, who Voss said had little time for page-long family pleas to spare Kayla.
“Kayla may not know that I retired this January,” an FBI-composed email from Carl Mueller explained on June 2, adding that after selling their auto repair shop the family had “limited resources.” ISIS told him to go back to work to earn money for the ransom demand.
“From this point on you will only speak about the objectives, SO NO MORE SENTIMENTAL SOB STORIES!” the hostage-takers responded.
The correct use of an American phrase suggested it was a western jihadi writing the messages, Voss said.
"'Sentimental sob stories' is a phrase that will only come from certain cultures," he said. "You begin to develop a cultural profile of who you're dealing with, because ultimately you want to track these guys down."
The FBI kept sending messages from the Muellers pleading for time. Carl Mueller began to suspect that the FBI was merely stalling with no intention of actually negotiating Kayla's release, and thereby angering ISIS.
In apparent frustration, ISIS sent an email that told her parents they wanted to know if the Muellers had “reached a SIGNIFICANT BENCHMARK with regards to the amount of CASH you have raised from the demanded sum.”
The FBI made no cash offer and instead had Mueller simply plead for more time and complain that the U.S. government wasn't helping.
“At one point I even said to the [FBI] team, I said if I got this email back I would be really mad,” Mueller told ABC News.
There were reasons for initially stalling including an unprecedented Joint Special Operations Command raid being planned for almost two months, held up for weeks before approval by the administration and by U.S. Central Command, current and former officials told ABC News. It culminated in an entire squadron from the Army's special mission unit Delta Force raiding a "desert pipeline prison" south of Raqqa over the weekend of July 4th.
But it was a "dry hole," a source said at the time -- the hostages had been relocated a few days earlier without overhead surveillance spotting the move.
"Yes, there was a period of deliberation I remember," a senior administration official told ABC News last week. "The intel wasn’t rock solid. It was compelling."
Assets had to be moved into the region, hundreds of operators were involved. A diversionary attack was launched nearby to draw fighters away from the prison site. It was also close to Damascus and the Assad forces had air defenses, the official recalled.
“We were working very hard to find the hostages. But after the July 4th raid, the trail ran cold," the senior official said.
On July 12, 2014, a week after the still-secret U.S. raid failed to rescue Kayla and the others, the hostage-takers’ tone grew suddenly more hostile in an email giving the Muellers 30 days to pay up.
"If you fail to meet this deadline we will send you a picture of Kayla’s dead body!" ISIS wrote. "This will be our FIRST act of revenge taken for the MISERABLY FAILED and unsuccessful attempt by your arrogant government to free their prisoners!"
Carl and Marsha Mueller were baffled.
"It was Kayla’s birthday, that was the deadline for them to kill her," Carl Mueller said. "So we immediately call our 'arrogant government' and say, 'What are they talking about?' And the response was, 'I don't know.' They had conducted a raid, a military operation. ISIS told us they were going to kill our daughter because they did that and their response was, 'No, we don't know anything about a raid.'"
The Delta Force raid was not publicly disclosed by the White House until six weeks later, after James Foley was beheaded in a shocking video by ISIS executioner Jihadi John.
Between July 12 and Sept. 19, 2014, the FBI composed and sent a dozen emails to the hostage-takers. In what would be the final email from ISIS to the Muellers while Kayla was still alive, and before she was taken by "caliph" Abu Bakr al-Baghdadi to be raped, they added a third demand.
"That is the immediate halt of ALL military activities by your government within and around the lands of the Islamic State," the email said, in the group's first reference they had made to the ISIS caliphate. "Kayla will not be released until these conditions are met."
Three days later, on Sept. 22, the U.S. commenced Operation Inherent Resolve in Syria, with more than 15,000 airstrikes since then.
Kayla Mueller was moved in August or September to the Shaddadi, Syria, house of a senior ISIS leader, Abu Sayyaf, and kept there for Baghdadi. ISIS tweeted on Feb. 6, 2015, that she had been killed in a Jordanian airstrike, which U.S. officials have denied. A building ISIS showed in a photo had been bombed much earlier as a barracks for fighters and arms depot, a counterrterrorism official has told ABC News. The White House confirmed Mueller's death a few days later without stating a suspected cause.
Retired FBI agent Voss said the final evidence that Kayla probably could have been ransomed was an email sent in February 2015, after her reported death, to Marsha Mueller containing three photos of Kayla's face after she was killed, dusty and sprinkled with tiny pieces of rubble, and one photo after her face was cleaned for burial, per Muslim custom. An accompanying note was respectful in tone.
"They respected her parents enough to send those photos. And they wouldn't have done that if her parents hadn't struck a chord with them. And if you can strike a chord, then you can create communication that's productive," Voss said.