A warning to students at an Indiana university in the US to take shelter after a gunman was seen on campus has been downgraded.

Purdue University Indianapolis sent out an alert to students on Twitter after an armed man was seen running to the university from a street close by, the Daily Mail reported.

“If you are on campus, trust your instincts and take safe shelter in the nearest available room,” the university said.

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“Lock the door if possible. Remain in place until the police or a campus administrator known to you gives the ALL CLEAR.

“The suspect is a 6ft 3ins or 6ft 4ins black male, seen wearing a black hoodie, blue jeans and a hat with yellow writing.”

They had since downgraded the warning, telling students to return to their exams.

“No suspect has been located, but IMPD continues to investigate and is lifting the “shelter in place.” Exams are on,” a Twitter post said.

In another tweet, the university said the incident had begun as an armed robbery at a nearby address.

“This situation began as an armed robbery in the 700 block of Blake Street. Police continue to chase leads.”

This came as a threatening email from a purported Muslim extremist forced the closure of every public school in the US city of Los Angeles.

By early Wednesday morning (AEDT), local police and the FBI had evacuated and begun searching more than 1000 schools.

IUPUI Alert! A subject with a gun has been reported running toward campus from the 700 Blake. Take safe shelter. Lock door. Follow official

— IUPUI (@IUPUI) December 15, 2015

Suspect is a black male, 6’3-6’4″ with a black hoodie, blue jeans and a hat with yellow writing. — IUPUI (@IUPUI) December 15, 2015

More to come.