Police spokesman Kese Smith says authorities have no immediate word of anyone being wounded.
Smith said authorities began receiving reports shortly after 2pm local time on Tuesday.
He said officers and a SWAT team were responding to the scene at Ben Taub Hospital, one of the hospitals that is part of the medical centre.
Ben Taub is one of the major trauma centres in Houston.
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Television images from helicopters showed dozens of employees leaving the hospital.
Some were attending to patients who appeared to have been taken out on gurneys.
Smith says officers and a SWAT team were responding to the scene at Ben Taub Hospital, one of the hospitals that is part of the medical centre.
More to come
