UPDATED 6:35 p.m.
Police and SWAT team members descended on on Ben Taub Hospital after multiple calls to police about gunfire inside the facility Tuesday afternoon.
The hospital says the incident has been "contained" and the all-clear was issued by day's end.
Houston Chief Art Acevedo says no one was injured and the shooter has not been found.
He also said gunfire has not been confirmed.
Police were first alerted of an incident at 1:58 p.m., Acevedo said. Evacuations and shelters-in-place took place immediately.
People said a bald white male was the shooter, but Acevedo said police have searched all floors and the basement and have not found a suspect. Authorities will also review security cameras.
Some at the Texas Medical Center facility have been led out of the building to safety -- but others are locked and huddled inside for safety.
At least a half-dozen current patients on stretchers have been wheeled outside and lined curbside, apparently for safety.
Roads around the hospital have been closed. People are being advised to stay away from the area.
The incident may have taken place on the hospital's second floor.
Ben Taub is owned by the Harris Health System. It is staffed by the faculty, residents and students from Baylor College of Medicine.
It is a Level I trauma center with nearly 600 licensed beds -- making it one the busiest trauma centers in the nation.
It also has a 24-hour psychiatric emergency room.