HOUSTON — A NASA contract worker took a handgun inside an office building Friday at the Johnson Space Center and fatally shot a hostage before killing himself, police said. A second hostage escaped with minor injuries.

The gunman shot himself once in the head more than three hours after barricading himself on the second floor of Building 44, which houses a laboratory.

The slain hostage was probably killed “in the early minutes of the whole ordeal,” Houston Police Capt. Dwayne Ready said.

None of those involved were immediately identified.

The man was believed to be an employee of Jacobs Engineering, which has a technical support contract with NASA.

“We understand it is one of our employees,” said John Prosser, the company’s executive vice president.

Roads within the 1,600-acre campus were blocked off during the confrontation. A nearby middle school also kept its teachers and students inside as classes ended.

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

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.

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.

President Bush was informed about the gunman as he flew back to Washington from an event in Michigan, White House spokeswoman Dana Perino said.