Story highlights U.S. didn't have "crystal clear idea" who hostages were, Pentagon spokesman says

Hostages "faced imminent mass execution," according to Pentagon

70 hostages were liberated in the operation, officials say

(CNN) About 70 hostages facing "imminent mass execution" were rescued and an American service member was killed in an operation against an ISIS-controlled prison in northern Iraq, the Pentagon said Thursday.

The commando became the first American combat death in Iraq since November 2011.

The hostages -- including more than 20 members of the Iraqi Security Forces, local residents and several ISIS fighters accused of spying -- were liberated in the helicopter assault, which involved U.S. special operations troops as well as Kurdish and Iraqi forces, U.S. officials said.

"There was not a lot of time," one U.S. official told CNN on condition of anonymity. "The threat of execution was imminent."

Mass graves dug inside the compound were spotted during surveillance, a U.S. official with direct knowledge of details of the raid told CNN. After the rescue, hostages said they had been told they would be executed after morning prayers.

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