"Kayla, along with our son and others were held for nearly two years and there were many opportunities along the way: several times when the captors reached out, several times when returning hostages brought sensitive information. And yet nothing was done to save our young Americans. So that's the part that deeply concerns me," Foley told ABC News in an interview, parts of which aired on "World News Tonight."
On Friday, ISIS claimed that Mueller, 26 -- whose name was mostly kept secret publicly because her captors had demanded a media blackout from her family in Prescott, Arizona -- was killed in a Jordanian airstrike on a building in Raqqa, Syria.
While Jordan did conduct airstrikes in Raqqa, ISIS has offered no evidence yet of her death, much less on that date or in a coalition bombing run.
Many counter-terrorism officials focused on ISIS remain deeply skeptical, while the Muellers in seclusion have signaled they do not know whether they should accept their daughter's death or not.
For much of 2013 and 2014, Diane Foley and hostage negotiators working for her family knew the likely locations of the western captives held by ISIS in Syria, even as they were moved from time to time. But the U.S. didn't launch a mission until July 3, when a squadron of operators from the elite Delta Force hit an oil refinery site near Raqqa, discovering it was a "dry hole."
"I certainly applaud the young, courageous soldiers who went on that mission. It was much too late and the intelligence obviously was poor -- it wasn't good enough for those courageous soldiers -- so they went in for nothing," Foley said.
A counter-terrorism official with knowledge of the planning and execution of the July 3 rescue operation said Diane Foley's assessment was correct.
"Washington took too long. We missed the hostages by two days. We could have had all the hostages -- all of them," the counter-terrorism official told ABC News last week, still upset over the outcome.
Since then, three American, two British, two Japanese and a Jordanian hostage have been executed in gruesome ISIS videos.
A spokesman for the National Security Council declined to address Foley's criticism directly but said the White House is conducting a review due this spring ordered by President Obama of the government's hostage policy, which generally falls under classified portions of National Security Presidential Directive-12.
"Family participation is an integral part of our review. Our goal has always been to do whatever we can within our capabilities and within the bounds of the law to assist families to bring their loved ones home," said NSC spokesman Alistair Baskev. "We have heard concerns expressed by some family members about their interaction and communication with U.S. government officials and the amount of information that can be shared about these efforts. We understand this is incredibly difficult and painful for the families and we appreciate their feedback."
Foley said her family's relationship with government officials "couldn't have been much worse, I'm afraid."
"We were always told to trust that Jim was the highest priority and that our government was doing all it could and I really found no evidence of that," she added. "Instead we were kind of told just to trust and that did not work out well... I shouldn't have trusted. We regret not having done more privately ourselves."