Police are investigating a potential hoax after finding no trace of a reported gunman on the campus of Yale University.

Students at the New Haven campus were told by university officials to "shelter in place" after an anonymous 911 caller told police that his roommate wanted to shoot up the college and was on his way to the main campus with an automatic rifle.

After the call, Yale went into lockdown and Swat teams scoured the grounds. The all-clear was given after a six-hour search.

Due to the Thanksgiving holiday, most students had already left the campus.

The phone call - now being investigated as a prank call - was made from a phone booth near campus.

The caller spoke briefly, stating that his friend had a long gun but did not identify himself as a student of Yale, said police spokesman officer David Hartman.

"All he really said was that his roommate was on his way to the university, to Yale University, to shoot people," he said.

Police chief Dean Esserman said: "He [the caller] sounded like a confused gentleman. I wouldn't describe his words as clear and concise."

People were interviewing several people who were unsure whether "they saw a police officer with a gun or a citizen with a gun," Esserman continued.