HOUSTON – A man with a handgun barricaded himself on the second floor of an office building Friday at NASA’s Johnson Space Center, causing police to evacuate some workers.

Authorities did not know the gunman’s motive, but described him as a man between 50 and 60 years old.

The building was evacuated immediately after reports of gunfire, but police said they did not know if there were any hostages. Capt. Dwayne Ready said officers have not been able to establish communications with the gunman.

Television reports said the man was an employee of Jacobs Engineering, a Pasadena, Calif.,-based subcontractor. Ready said he did not know who the man was. Phone messages left at Jacobs Engineering were not immediately returned.

Ready said two shots were fired, but he did not know whether anyone had been wounded.

SWAT officers surrounded Building 44, which houses communications and a laboratory. Roads within the 1,600-acre campus were blocked off. A nearby middle school also kept its teachers and students inside as classes ended.

Christine Reichert, space station flight controller at the space center, said employees were initially told to stay in their buildings. That restriction was lifted a few hours later.

NASA employees and contract workers were kept informed of the situation by e-mail.

Michael Zolensky, who studies cosmic dust at the space center, said workers were gathered around a television watching news reports of the situation.

Doors to Mission Control were locked as standard procedure.