Updated at 3:10 p.m. Monday: Revised to show the all-clear was given.

A building in the UT Southwestern complex was evacuated by hospital police after a letter threatening a shooting was discovered Monday morning.

Officials haven't identified "immediate threats," but the building was emptied as a precaution, according to a written statement from the hospital.

Operations returned to normal about 1 p.m., the hospital said.

A letter threatening active shooting was discovered at #UTSouthwestern this morning. University Police are actively investigating the threat and patrolling campus. No immediate threats have been identified. In an abundance of caution, one building on campus has been evacuated. — UT Southwestern News (@UTSWNews) May 21, 2018

UT Southwestern didn't specify which building was evacuated, but said it was a non-clinical academic office building. The hospital said its police force was investigating the threat and patrolling campus.

The hospital said the location and contents of the letter were not disclosed because they were part of the investigation.

A Dallas police spokeswoman said the agency had not been contacted by UT Southwestern.

No other details were immediately available.