A U.S. rescue operation earlier this summer failed to free American journalist James Foley and others being held by ISIS in Syria, a U.S. official told CNN on Wednesday.

The Pentagon confirmed a rescue attempt, but did not say whether Foley was among those U.S. officials were hoping to free.

"Unfortunately, the mission was not successful because the hostages were not present at the targeted location," Pentagon spokesman Rear Adm. John Kirby said.

A U.S. official, speaking on condition of anonymity, told CNN the mission was to rescue Foley and other American hostages being held at an undisclosed location.

The news came the same day that it was revealed Foley's family was contacted a week ago by his captors, who said in a rage-filled email the journalist would be executed.

A video released by ISIS on Tuesday showed Foley being beheaded and warned that a second American would be killed if the United States did not end its military operations in Iraq.

"The message was vitriolic and filled with rage against the United States. It was deadly serious," said Philip Balboni, CEO of the online publication GlobalPost.

"Obviously, we hoped and prayed that that would not be the case ... Sadly, they showed no mercy."

In the video, which CNN is not showing, Foley is seen on his knees and a man cloaked in black stands behind him.

Foley is then executed.

The video of his killing also shows another U.S. journalist, believed to be Steven Sotloff. The militant in the video, who speaks English with what sounds like a British accent, says the other American's life hangs in the balance, depending on what President Barack Obama does next in Iraq.

'Jim was innocent'

Foley, a freelance journalist, was on assignment for GlobalPost when he disappeared on November 22, 2012, in northwest Syria, near the border with Turkey.

ISIS, the militant group seeking an Islamic caliphate stretching from Iraq into Syria, has claimed credit and U.S. intelligence said that the video is real.

"Jim Foley was an incredibly brave journalist and an incredibly brave man, right to the horrible end of his life. We are devastated by his loss," Balboni told reporters. "Jim represented the best of the profession of journalism and what it can be and what it should be in its finest hour."

In the video posted on YouTube, Foley reads a message, presumably scripted by his captors, that his "real killer'' is America.

"I wish I had more time. I wish I could have the hope for freedom to see my family once again," he can be heard saying in the video.

His parents, flanked by one of Foley's brothers, talked to reporters Wednesday.

"Jim was innocent and they knew it," Diane Foley said. "They knew that Jim was just a symbol of our country."

His father, John, broke down several times.

"We beg compassion and mercy" for those believed to be holding the other American journalist shown in the video. Sotloff, a contributor to Time and Foreign Policy magazines, was kidnapped at the Syria-Turkey border in 2013.

"They never hurt anybody," John Foley said. "They were trying to help. There is no reason for their slaughter."

Foley had previously been taken captive in Libya. He was detained there in April 2011 along with three other reporters and released six weeks later.

Afterward, he said that what saddened him most was knowing that he was causing his family to worry.

His parents talked about asking him why he wanted to return to conflict zones.