US commandos searching Syria for executed hostages' remains: Officials

The operation was launched based on new intelligence from captured ISIS members.

Three officials briefed on the ground searches by U.S. commandos -- the first real effort to recover the remains of two American journalists and two American humanitarian aid workers killed from 2014 to 2015 -- said they were undertaken on the basis of new intelligence from two ISIS members from London captured last month by Syrian Kurds.

“Intel people are digging hard and directing ground guys to locations,” a counterterrorism official who is not authorized to speak publicly told ABC News.

Two other counterterrorism officials confirmed to ABC News that ground searches have begun and suggested the efforts are limited so far but could be expanded. The remains are believed to be spread out over several sites.

Alexanda Kotey and El Shafee Elsheikh are two of the most important ISIS figures ever captured on the battlefield. U.S. commanders at Joint Special Operations Command and CIA have had a policy of carrying out targeted killings with armed drones of the terrorist group's senior leadership rather than more risky attempts to capture them.

The two men are being interrogated by U.S. officials and are providing information about burial sites of American hostages they tormented and executed while running a kidnap and ransom operation for ISIS, which began in 2013. Ransom demands were halted after a July 4, 2014 failed Delta Force rescue raid and the subsequent U.S.-led coalition air campaign against ISIS.

The U.S. Hostage Recovery Fusion Cell has contacted families of the hostages who died -- some of whom were shown having their heads cut off by Jihadi John on video -- many of whom welcomed the capture of the pair and U.S. efforts to find the remains of their loved ones.

"I am grateful for all these efforts but bringing these two jihadists to justice gives us more hope and solace than the return of Jim's remains," she said.

"I don't know what happened to Kayla. I don't where Kayla is. I need her home," said her mother, Marsha, on Friday. "She belongs here."

Kayla's father, Carl Mueller, said he appreciates that any effort to recover the remains of his only daughter, whose death is still a mystery. The Muellers recently pressed U.S. hostage recovery officials in a Washington briefing to reveal how their daughter died and were told specific details have eluded investigators.

"Unlike the other families, we don't have the gruesome execution video,” Carl Mueller told ABC News. “We need to know she's gone.”

"I would like anyone who has information about Kayla to share that with us," her mother said.

That includes Kotey and Elsheikh, who may know how she died and where she is buried.

According to J.M. Berger, author of “ISIS: The State of Terror,” Kotey and Elsheikh are the most important ISIS figures captured in several years.

"This is a really big development," Berger said. "ISIS has provided an incredible documentation of its atrocities and we haven’t seen anyone held to account for that. It would be hard to see how these guys could get off. People need to see justice being done."