MOUNTAIN VIEW, Calif. (AP) _ A Lockheed equipment technician may have committed suicide or ''just done a dumb thing'' when he put on a sealed helmet pumping pure nitrogen, a company spokesman said Thursday.

Ronald Johnson, 29, an eight-year veteran of the Air Force, died at the National Aeronautics and Space Administration's Ames Research Center on Tuesday while doing routine tests prior to a U-2 jet flight, said Lockheed spokesman Nick Duretta said.

Nitrogen is used instead of oxygen to test for leaks and proper gas flow because of the fire danger associated with oxygen, but testers are not supposed to wear the helmet, Duretta said.

Johnson was not wearing any other pilot's gear at the time he was found by fellow worker Gil Caganaut.

The helmet has a ''bladder'' inside that acts as a seal, preventing air from coming in. It also has an anti-suffocation valve, but that has no effect if nitrogen is present in the helmet, Duretta said.

''He was found with the helmet on, with nitrogen flowing into the helmet,'' said NASA spokesman Larry King. ''For some reason, he put on the helmet. Nobody knows why.''

NASA and Lockheed are conducting an investigation and have reached no conclusion. Results of an autopsy by the Santa Clara County coroner is pending.

Johnson ''had gone through the procedure a number of times,'' Duretta said. ''One would think that (from Johnson's Air Force experience and Lockheed training) he was aware of the dangers of putting the helmet on with nitrogen coming through.''

Duretta said it is standard practice to test helmets and suits before every flight, and ''they fly fairly frequently up there.''

All technicians are aware of the dangers of nitrogen, he said.

''You take a big whiff of that (nitrogen) and it acts pretty quickly,'' he said. ''One hundred percent nitrogen can kill you in a matter of seconds.''

He said suicide is ''one of things that would be considered'' by the investigative team.

''It could have been suicide, or he may have just done a dumb thing and couldn't get it off in time.''

Johnson was stationed at Beale Air Force Base near Marysville before joining Lockheed on Jan. 13. He lived alone in San Jose and was the divorced father of a 4 1/2 -year-old girl, Duretta said.

Lockheed has had a contract with NASA since 1982 to maintain the U-2 jets at the center, and had never had an employee die under such circumstances, Duretta said.