The U.S. government obtained intelligence on the possible location of American captives held by ISIS in Syria last year, but Obama administration officials waited nearly a month to launch a rescue mission because of concerns that the intelligence wasn’t conclusive and some of it had come from a foreign service, U.S. and British officials told The Daily Beast.

British officials, as well as private security contractors, said they were frustrated by Washington’s hesitance to give the go-ahead for a rescue attempt, which eventually was carried out on July 4, 2014, by which time the hostages had been moved. The following month, ISIS began beheading its American and British prisoners in a series of grisly Internet videos.

Toward the end of May, the British government had identified two or three locations in and around the Syrian city of Raqqa, the de facto capital of the so-called Islamic State, where the militants had moved hostages during the previous weeks and months. But the British were not absolutely sure in which location the Westerners were held. The captives included American journalists James Foley and Steven Sotloff, as well aid worker Kayla Mueller. The information—based on debriefings of European captives who had been released, satellite and drone surveillance, and electronic eavesdropping—was not definitive in May.

Then, in early June, London had a “positive identification and that information was shared with Washington,” said a British source. The delay of nearly a month before the rescue bid was mounted remains a source of bewilderment for British officials.

But a U.S. official said that inside the White House, Obama’s senior national-security advisers were not willing to base a raid on intelligence developed by a foreign service. “The issue was that they didn’t trust it, and they wanted to develop and mature the intelligence, because it wasn’t our own,” said the U.S. official, who asked to remain anonymous when discussing sensitive hostage-rescue efforts.

“They got the information. They just didn’t trust it. And they did sit on it, there’s no doubt about that,” the official said.

Bernadette Meehan, National Security Council spokesperson, told The Daily Beast: “U.S. forces conducted this [rescue] operation as soon as the president and his national-security team were confident the mission could be carried out successfully and consistent with our policies for undertaking such operations.”

But Diane Foley, the mother of James Foley, who was the first American to be shown murdered on camera, also raised questions about the timing of the rescue effort, telling The Daily Beast that French officials had developed information about the hostages’ location as early as March, but that the U.S. government didn’t act on it.

“That was part of our frustration,” she said. “The State Department said they were connecting with the French and everybody at the highest levels.” And yet, there was no movement on the U.S. side. “Very specific information was available as early as mid-March. And that’s what’s been so tough for us as families, because apparently they were held in the same place all those months,” Foley said.

Last August, after the administration disclosed the failed raid, National Security Adviser Ben Rhodes pushed back against the suggestion that the United States hadn’t done enough to win the release of the American captives.

“I can assure you that we have done everything that we can possibly do to try to bring home our hostages,” Rhodes said during a press briefing on Martha’s Vineyard. “It’s an incredibly difficult circumstance in a place like Syria, again, where you have such a violent conflict raging. But we’ve used all of our military, intelligence, diplomatic resources that we can bring to bear to try to pull a thread to find out where our hostages are; to try to rescue them when we saw an opportunity; to try to work with any country that might have any means of locating them.”

U.S. intelligence agencies had also debriefed European hostages who were released after their governments made ransom payments. The Washington Post reported last August that the president had authorized a rescue attempt “earlier this summer,” citing a senior administration official.

“We had a combination of... intelligence that was sufficient to enable us to act on it,” the official told the Post, and the military moved “very aggressively, very quickly to try and recover our citizens.”

Rescue missions are inherently dangerous, and recent attempts have ended in the death of the hostages. But family members of the Americans held by ISIS, including the parents of Foley and Sotloff, as well as Foley’s brother, have said the U.S. government did not do enough to rescue their loved ones.

Responding to the general criticism of the United States being slow to act, President Obama told BuzzFeed this week, “I don’t think it’s accurate then to say that the United States government hasn’t done everything that we could. We devoted enormous resources—and always devote enormous resources—to freeing captives or hostages anywhere in the world, and I deployed an entire operation at significant risk to rescue not only [Mueller], but the other individuals that had been held, and probably missed them by a day or two, precisely because we had that commitment.”

The question of U.S. rescue efforts has taken center stage again with the death of Mueller, a 26-year-old aid worker who was taken captive in Syria in 2013. U.S. officials and Mueller’s family confirmed this week that she was killed while held hostage by ISIS, although the cause of her death has not been determined.

Questions are now being asked of Mueller’s then-boyfriend, who was with her when they and two others were snatched by ISIS after visiting a Doctors Without Borders hospital near Aleppo on Aug. 4, 2013. He is a Damascus-born freelance photographer who had also undertaken some projects for the Danish Refugee Council and worked for a time as a staff reporter at Reuters. The pair met in Cairo, according to a former colleague who worked with the man.

After the pair were detained, ISIS let Mueller’s boyfriend go—a move that later prompted some speculation that he might have been involved in the abduction. But those fears were dispelled when, at great risk to himself, he went to Raqqa to try to negotiate her freedom. He claimed to be her husband, but Mueller reportedly told her captives that she wasn’t married. It’s not clear whether she was aware that her boyfriend had returned for her. But he was seized and held for two months, from December 2013 to January 2014, before being set free again.

“He was besotted with [Kayla],” said the man’s former colleague. “There was talk before her capture of an engagement.” In an email to The Daily Beast, the photographer declined to be interviewed, writing, “let me take my time to understand what happened and to bring myself all together.” He has also declined interview requests from private security contractors working on hostage cases.

Writing under the pseudonym Omar Alkhani on Facebook, the man has posted romantic missives to Mueller. In a post apparently written after learning of confirmation of Mueller’s death, he bid her farewell. “Thank you for coming into my life and giving me joy, thank you for loving me and receiving my love in return,” he wrote in one post. “You were everything I wanted. You were so beautiful and charming, and you supported me in everything I did, even if it was extremely stupid.”

“Alkhani” generally avoided writing about her after her capture, but in September 2013 posted, “My heart is in your detention cell/it lives your pain all day/you are not alone/you are not alone.”

On Wednesday, reports surfaced that Mueller may have been given as a bride to an ISIS fighter. But British sources questioned the claim, saying it was the first they had heard of it and it didn’t crop up in any “chatter they were monitoring” or in communications between London and Washington. “I find the report dubious: She had potential monetary value for them as far as they were concerned,” says a British source.

ISIS claims that Mueller was killed last week in a Jordanian airstrike conducted in revenge for the group burning alive a Jordanian pilot. The Jordanians dismissed the allegation as ISIS “spin,” and U.S. officials have said they have no information on how Mueller died.

ISIS showed a photograph of a bombed-out building where it claimed Mueller was being held. It was a known hostage location, said European officials, who have shared information on Western hostages. That information was also shared with the Jordanians.