Justin Mohn bought the gun the day before the murder, according to the Bucks County district attorney.
Islamic State extremists released a video showing the beheading of British aid worker David Haines, who was abducted in Syria last year, and British Prime Minister David Cameron late Saturday condemned his slaying as "an act of pure evil."
President Barack Obama says NATO members unanimously agree on the need for immediate action against Islamic State militants because they pose a threat to members of the alliance.
Islamic State extremists on Tuesday released the video showing freelance reporter Steven Sotloff's beheading, two weeks after the emergence of a video showing the killing of journalist James Foley.
An Internet video posted online Tuesday purported to show the beheading of U.S. journalist Steven Sotloff by the Islamic State group, which called it retribution for continued U.S. airstrikes in Iraq.
Joe Torres reports on the beheading of James Foley