ISIS had recently threatened to kill U.S. journalist James Foley to avenge airstrikes the United States has conducted in Iraq, a senior U.S. official told ABC News.

The White House had been aware of the threat prior to the release of a video Tuesday night that appears to show the beheading of Foley and warns that militants will carry out a similar act against U.S. journalist Steven Sotloff, who went missing in August, the official said.

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At least one news organization said it notified the White House about threats it received regarding Foley. Phillip Balboni, CEO and founder of the online news service Global Post, told ABC News' Brian Ross that the company received an e-mail on Aug. 13 demanding ransom and threatening to execute Foley. Balboni said the threat was shared with the White House.

Balboni said his news organization began receiving threats from Foley's captors via e-mail as early as last fall. The final e-mail Balboni received was "vitriolic and filled with rage against the United States," he said.

"It was deadly serious," Balboni told reporters.

(Nicole Tung/AP Photo)

Balboni noted the e-mail messages changed dramatically in tone after the U.S. began airstrikes in Iraq.

"The situation changed when the bombing began," he said. "That made everything much more difficult."