U.S. interrogators are questioning the wife of a senior ISIS leader for information on American and Western hostages that the group has held and either ransomed or killed.

U.S. officials told The Daily Beast that among the questions being posed to Umm Sayyaf, who was married to Abu Sayyaf, an ISIS figure killed in a U.S raid in Syria on Friday, is what she knows specifically about the kidnapping of Americans, a senior defense official told The Daily Beast. The terror group beheaded several American and British hostages in a series of grisly online videos. And its last known American hostage, aid worker Kayla Mueller, died while in ISIS custody in a manner that has never been fully explained.

Officials believe that Abu Sayyaf, whom the U.S. has called the group’s chief financial officer, held key information about the group’s kidnapping operations. They also suspect that his wife may have played an operational role in ISIS and likely knows something about how the group captured, held, moved, when they were killed, and in some cases how ISIS ransomed its hostages. In addition, interrogators will want to know if future kidnappings are planned.

U.S. officials had hoped that they could learn such details from Abu Sayyaf himself, but in the absence of that, “we want to know what she knew,” the defense official explained, referring to his wife.

Charles Regini, a former top FBI hostage negotiator, told The Daily Beast even though ISIS’s American hostages are dead, the bureau is still investigating.

“The cases remain open as FBI kidnap-murder investigations until the perpetrators are identified,” Regini said. “The FBI would be seeking to gain specific details as to Sayyaf’s potential involvement in the kidnap-murders, as well as the names of others and details of their involvement, to be used as evidence in a criminal prosecution.”

According to U.S. estimates, at its peak, ISIS made $25 million in one year through ransom payments.

A former U.S. official, who had no specific knowledge of the raid that captured Umm Sayyaf, said that President Obama wouldn’t have authorized a risky ground operation unless he thought it could yield vital information that could help save innocent lives. U.S. intelligence agencies have already been collecting a significant amount of information about ISIS finances and communications—Abu Sayyaf’s alleged bailiwicks. More information of this kind wouldn’t be valuable enough to risk American military casualties, the former official said.

That helps explain why American forces undertook a mission to capture an ISIS official whom terrorism experts have said wasn’t so senior that he couldn’t be easily replaced. As it turns out, who Sayyaf was is less important than what he and his wife may have known about hostages.

Umm Sayyaf is being interrogated in Iraq by the FBI’s High-Value Detainee Interrogation Team, according to House Intelligence Committee ranking member Adam Schiff, D-Calif., and a senior administration official who spoke anonymously because he was not authorized to speak publicly.

The FBI team is deployed when the government needs to extract information quickly from a detainee and may or may not present the information in court. The interrogators cannot torture the detainee, but they can question her outside of the presence of a lawyer and following different rules than if the subject were being questioned inside the United States.

“There’s a belief that [Umm Sayyaf] was active within ISIL and had responsibilities within ISIL apart from being a spouse of Abu Sayyaf,” Schiff told The Daily Beast on Sunday. ISIL is short for Islamic State of Iraq and the Levant, another name used to describe ISIS.

She may have had a possible “operational role,” he added.

Army Colonel Steven Warren, a Pentagon spokesman, told reporters Monday that U.S. officials would not release Abu Sayyaf’s proper name. Abu Sayyaf, who defense officials have said was a Tunisian national, is a nom de guerre. (Generally translated as “Father” or “Bearer of the Sword.” Umm Sayyaf means “Mother of the Sword.”)

ISIS isn’t believed to be holding any more American hostages. It does have at least one remaining British captive, John Cantlie. But it’s not clear that the group has many other Western blasted. However, it has staged horrific beheadings of Syrians and Libyans and has inflicted a reign of terror on Muslims and Christians in Iraq.

But the former official said that even retrospectively, there is a lot that intelligence analysts can learn about how ISIS collected ransoms, where they held hostages, and how they moved them around Syria—all clues that could help rescue any future captives.

U.S. officials are hoping to reconstruct a timeline of each kidnapping and learn things like how often hostages were moved, where to, how, and why. "We want to understand as much as we can about how they conduct hostage operations," the defense official said.

Over the weekend, there were unconfirmed reports that Abu Sayyaf and his wife may have had specific information about Mueller, the only American woman ISIS has held hostage, and how she was treated and eventually died.

A spokesperson for the Mueller family told The Daily Beast that they had no immediate comment but that the family is monitoring the news as it develops.

Umm Sayyaf, an Iraqi national according to defense officials, is being questioned in Iraq, What will happen to her once that questioning is completed remains unclear.