U.S. Attorney General Eric Holder has announced that the Justice Department is conducting a criminal investigation into the killing of journalist James Foley.
Foley was decapitated in Syria by militants of the Islamic State, who released a video of the execution and called it revenge for U.S. air strikes against its fighters in northern Iraq.
The masked individual who executed Foley spoke with a British accent, leading investigators to believe he was a British national who joined the militant group.
Washington acknowledged on August 20 that special forces were sent to Syria earlier this summer to try to locate and free Foley and other American hostages held by the Islamic State, but failed to find them.
Foley's employer, GlobalPost, revealed that the Islamic State demanded a ransom to release the captured journalist.
GlobalPost CEO Philip Balboni confirmed there was an initial ransom demand for 100 million euros (some $132 million) from the captors.
U.S. President Barack Obama has said he was appalled by Foley's execution and called the Islamic State a "cancer."
Obama's national security adviser Ben Rhodes said the United States would not be restricted by geographical boundaries when it came to protecting U.S. citizens and added the United States did not rule out "additional action against [Islamic State] if it becomes warranted."
U.S. warplanes, meanwhile, continued to strike Islamic State positions in northern Iraq on August 21.
The U.S. military's Central Command said in a statement its forces "continued to attack [the Islamic State] terrorist in support of Iraqi Security Force operations, using fighter and attack aircraft to conduct six air strikes in the vicinity of the Mosul dam."
The statement said the U.S. military had carried out 90 air strikes in Iraq since August 8, including the latest attacks.
Meanwhile, officials said U.S. Defense Secretary Chuck Hagel and top General Martin Dempsey will hold a press conference on August 21.
The Islamic State occupies areas across northern Syria into western and northern Iraq.
The militants had seized the strategic Mosul dam in northern Iraq but Kurdish forces backed by U.S. bombing raids have recaptured the dam.
With reporting by Reuters and AP