SAN FRANCISCO (AP) — The FBI has confirmed it is investigating a recent hacking attack at Sony Pictures Entertainment, which reportedly caused major internal computer problems at the film studio last week.

Sony’s corporate email and other internal systems were knocked offline, according to reports by Variety and other trade publications. Sony workers reportedly saw a message appear on their computer screens that said “Hacked by #GOP,” which may be the initials of a group calling itself Guardians of Peace. Copies of some unreleased Sony films such as “Still Alice,” ‘’Annie,” ‘’Mr. Turner,” and “To Write Love on Her Arms” are now being distributed on unauthorized file-sharing websites, although a direct connection to the hacking hasn’t been confirmed.

The FBI said in a statement that “the targeting of public and private sector computer networks remains a significant threat.” Sony Pictures, based in Culver City, California, wasn’t immediately available for comment.