The U.S. intelligence community believes that American hostage Kayla Mueller may have been given as a bride to a fighter with the Islamic State of Iraq and Syria, or ISIS, during her captivity, CBS News has learned. That belief is based on an intelligence assessment, but has not been established as a hard fact.

The White House confirmed the Arizona woman's death Tuesday.

She is the fourth American to die in the hands of the extremist group.

In response to the question of Mueller being given as a bride, White House National Security Council spokesperson Bernadette Meehan said: "Out of respect for Kayla's family, we have not shared details regarding her time in ISIL captivity. ... Reports that have been published by certain news outlets regarding Kayla's time in ISIL captivity are speculative and unproven at this time. The U.S. Government continues to analyze any information regarding Kayla's captivity but has not corroborated any of the multiple theories regarding the conditions throughout the duration of her captivity."

Lawmakers on Capitol Hill said all the circumstances around Mueller's captivity and death underscore the need for Congress and the president to agree on a framework for a battleplan to defeat ISIS in Iraq and Syria, CBS News' Major Garrett reports. Congress could receive President Obama's formal proposal on that as early as Wednesday.

Mr. Obama knew Mueller's inspiring story, one the country is just coming to fully appreciate. It's one of the reasons the president called her parents before her death was publicly confirmed and why he issued such a lengthy statement of tribute.

The president also revealed for the first time that Mueller was one of the hostages U.S. forces tried to rescue in a failed covert operation in Syria in July.

Mr. Obama told Buzzfeed News the effort to free U.S. hostages is relentless.

"I deployed an entire operation -- at significant risk -- to rescue not only her but the other individuals that had been held and probably missed them by a day or two," Mr. Obama said.

Rep. Paul Gosar, whose district includes the Mueller family's home, told The Arizona Republic that there were several failed attempts to rescue Mueller.

In one, a man pretending to be Mueller's husband appeared at the Syrian terrorist camp where she was being held and demanded her release. But Mueller, unaware it was a rescue mission, denied she had a husband and unknowingly botched a chance at freedom.

The White House is reviewing how to improve communication with families of Americans captured abroad, but the president stood by the longstanding U.S. policy banning the payment of ransom for any hostage.

"Once we start doing that, not only are we financing their slaughter of innocent people and strengthening their organization, but we're actually making Americans even greater targets for future kidnappings," Mr. Obama said.

Intelligence agencies confirmed Mueller's death after reviewing a photo ISIS sent the Mueller family. While U.S. officials could not determine the cause of death, they denied ISIS claims she was killed in a Jordanian airstrike.

"Let's not forget in whose hands this woman died," said Rear Adm. John Kirby, before referring to ISIS by ISIL, or the Islamic State of Iraq and the Levant, "and let's not forget who's ultimately responsible for it: ISIL."

On Capitol Hill, lawmakers praised Mueller's efforts to aid Syrian refugees and her remarkably positive attitude throughout her captivity.

"The letter that she sent home showed incredible grace, incredible courage, incredible strength in the face of an enormous evil," said Sen. John Thune, R-South Dakota.