Fate of ISIS’s US Hostage Unknown, Experts ‘Skeptical’ of Terror Group’s Claim

— -- Three tense days after ISIS claimed its American hostage, aid worker Kayla Mueller, was killed in a Jordanian airstrike, the terror group has not presented any hard evidence to back up its claim – a failure that has prompted doubts in ISIS’s story and inspired the Mueller family to cling to long-held hope that she’s alive.

A statement from ISIS that circulated online Friday said Mueller, 26, was killed when Jordanian bombs hit a building outside Raqqa, Syria. The statement used Mueller’s full name, which had not been made public.

But though ISIS produced pictures of the bombed-out building, it did not provide any evidence that Mueller was there. Previously, ISIS has shown photographs of casualties on both sides of the Syrian conflict to brag about killings or memorialize the deaths of its own fighters. The relative photographic silence when it comes to Mueller’s death has been deafening, according to former U.S. officials.

“I think we should be skeptical for several reasons,” Retired Lt. Gen. Mike Barbero, former deputy commander of U.S. Forces-Iraq, told ABC News today. “These are masters of using video and social media to exploit, recruit [and there’s been] no evidence from them… I think they’d show if it was the case, they would show some sort of evidence.”

“My sense is that they’ve not shown us more because they have nothing to show us,” said Brad Garrett, a former special agent with the FBI and ABC News consultant. “The whole thing is completely fishy that what they’re saying is on the up-and-up… I have complete doubts about the truthfulness of this story.”

Matt Olsen, former Director of the National Counterterrorism Center, said it was “quite relevant” that ISIS hasn’t tried to show proof of Mueller’s death.

“ISIS has not hesitated in the past to show gruesome images of executions and death, so at this point I think we need to be quite skeptical of any claim that they’ve made that Kayla Mueller was killed… until such a claim is fully corroborated,” said Olsen, who is also an ABC News consultant.

But Barbero said it would be difficult to determine for sure if ISIS is lying as well.

“It would be absolutely hard, if not impossible to verify one way or another unless you had some physical evidence or some source on the ground to determine whether she had been killed or not,” he said.

U.S. counter-terrorism officials have bemoaned the lack of on-the-ground intelligence available in Syria.

Mueller’s parents, who live in Arizona, are clinging to hope in the absence of certainty, according to family friend Todd Geiler.

“After [ISIS’s claim] our whole world kind of got turned upside down,” Geiler told ABC News’ Phoenix affiliate KNXV. “This ordeal has been going on for so long… [Mueller’s parents] are weary, they’re tired, but they’re also very, very hopeful and prayerful that their daughter will be returned to their safe keeping.”

The U.S. government said Friday that it had seen no evidence to corroborate ISIS’s claim and today U.S. officials declined to comment further to ABC News.

ABC News' Lee Ferran contributed to this report.