There was no word on whether anyone had been shot or if the man had taken any hostages. NASA officials were unable to confirm whether all of their employees had made it out of the Houston building safely.

"We do not know if there is another person inside with this suspect," Houston police captain Dwayne Ready said at a press conference.

"We have not established communication. And if he's alive or dead, I simply don't know."

A NASA employee told police he heard two shots fired and saw a man wielding a handgun in building 44 at around 1:40 p.m. (18:40 GMT), Ready said.

Police do not know whether anyone was hit by the gunfire or if it was merely directed at the floor or ceiling.

Nobody has reported seeing any victims inside the building and paramedics have not treated anyone who fled the building after the shooting began.

Ready was unable to confirm reports that the gunman was a NASA contractor who walked into a conference room and told everyone to leave.

NASA officials said mission operations were not interrupted by the incident. An e-mail was immediately sent out to all employees telling them to shelter in place.

Once officials realized the situation had been contained to the one building, a second e-mail was sent out telling employees they could go home whenever they wished.

"We have a standard set of security rules that do include random vehicle searches," NASA spokeswoman Eileen Hawley said at the press conference.

"Certainly, I would believe that our security and our senior leadership is going to take a very close look at this incident, and see if there was anything that we should have done or could have done differently."

The sprawling Johnson Space Center campus is home to the mission control center for space shuttle missions.

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