Workers evacuated a building at NASA's Johnson Space Center and police surrounded it on Friday after reports that a man with a gun was spotted and shots were fired, authorities said.
NASA spokeswoman Brandi Dean said it was not known if anyone was hurt and Kelly Humphries, another spokesperson, said it was not yet known if there had indeed been gunshots.
The incident added to jitters across America after a student gunman killed 32 people Virginia Tech university, in the worst shooting rampage in the nation's modern history.
Television reports said the gunman at the Johnson Space Center was barricaded inside building that houses a communication system, but police spokeswoman Johanna Abad could not confirm that.
Abad said police went to the center in the Houston suburbs after receiving reports of a gunman and that shots had been fired. She said Special Weapons and Tactics, or SWAT, teams had been sent to affected building.
NASA said police were surrounding the building and employees in nearby buildings had been asked to stay in their offices.
(Additional reporting by Erwin Seba, Anna Driver and Eileen O'Grady)