US Attorney General Eric Holder has said those responsible for journalist James Foley's murder will be held accountable "one way or the other".
Holder warned: "Those who would perpetrate such acts need to understand something - this Justice Department, this Department of Defense, this nation, we have long memories and our reach is very wide.
"We will not forget what happened and people will be held accountable one way or the other."
He stressed that the open criminal investigation into Foley's death will be pursued "vigorously".
A British family have told ITV News of their devastation after discovering a close relative is fighting with the Islamic State in Iraq.
A French journalist who was held hostage with James Foley in Syria has told ITV News it was "devastating" to see his murder.