WASHINGTON (CNN) -

U.S. special operations units were sent into Syria this summer to rescue American journalist James Foley and other hostages held by Islamic militants, a U.S. official told CNN.

Several dozen of the most elite U.S. commandos from units like Delta Force and Navy SEAL Team 6 flew in aboard helicopters but couldn't find the hostages, including Foley, whose grisly execution was captured on video and released this week by ISIS, the terror group that refers to itself as the Islamic State.

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

It's the latest revelation about Foley's final days in the hands of ISIS, which taunted his family in an e-mail a week ago, saying he would be killed.

"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, which employed Foley.

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

Balboni told the Wall Street Journal that the captors originally demanded a ransom sum of 100 million euros ($132.5 million) from Foley's family and GlobalPost.

Then came the message sent to Foley's family last week. "There was no demand," Balboni said.

Obama: ISIS is a 'cancer'

In the video, which CNN is not showing, Foley is seen on his knees as a man cloaked in black, his face covered, 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.

But the threat did little to curb U.S. military operations in Iraq, with American warplanes carrying out at least 14 airstrikes against ISIS targets.

Calling ISIS a "cancer," Obama said the United States "will continue to confront this hateful terrorism and replace it with a sense of hope and civility."

Several ISIS operatives were killed in the special operation earlier this summer that tried to rescue Foley and others, the U.S. official said. No U.S. personnel were killed, but one was slightly wounded. Fighters jets and surveillance aircraft provided overhead protection to the troops.

Foley's father: They showed no mercy

Messages from Foley's captors began last fall, Balboni of GlobalPost said. Foley, a freelance journalist, was on assignment when he disappeared on November 22, 2012, in northwest Syria, near the border with Turkey.

"The captors never messaged a lot. There was a very limited number with a very specific purpose. ... They made demands," Balboni said.

Some messages were political and some were financial.

Then came the final message last week, without any demand.

Foley's family, according to Balboni, responded in an e-mail, pleading for mercy and asking for more time.

They did not hear back.

The captors showed no mercy, Foley's father, John, told reporters on Wednesday, breaking down in tears.

Foley's family appears to have been among the journalist's final thoughts.

In the execution video posted Tuesday to YouTube, Foley reads a message, presumably scripted in part, if not all, by his captors. "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.