WASHINGTON (AP) _ Despite security concerns, scientists at federal weapons labs must be allowed freedom to exchange ideas within the scientific community, the director of the Los Alamos National Laboratory said Thursday.

The laboratory in New Mexico has been the focus of an espionage investigation following allegations that China may have stolen information from the lab about a critical nuclear warhead in the 1980s.

Some members of Congress have criticized the open ``campus-like atmosphere'' at the 47-square-mile Los Alamos facility, one of the government's top-secret weapons research labs. It is managed by the University of California.

But John C. Browne, who became director at Los Alamos in 1997, said his scientists ``know how to draw the line'' when talking to outsiders and should be given some leeway.

``It is a campus. It's a campus behind a fence,'' said Browne during a hour-long meeting with reporters. ``We can't just put everybody behind a fence and lock them up and let them do their job.''

Browne said members of a congressional intelligence panel raised concerns about scientific freedom during a recent visit to Los Alamos. ``They left better informed, but not convinced.''

Congressional critics have said security at Los Alamos and other national weapons labs has been insufficient for years, with adequate attention placed only recently on improving counterintelligence activities.

Browne said he didn't ``want to defend what was going on 10 to 20 years ago.'' Still, when he first learned that China likely obtained information about a key nuclear warhead, the W88, from Los Alamos he found it hard to believe. It felt ``like a hole in the pit of your stomach,'' he said.

He said he's still not convinced that a Los Alamos computer scientist, who has been the target of an FBI investigation for three years, gave the Chinese the W88 information. The scientist has not been charged, but was fired for security violations earlier this year.

``It's still not clear from what I read whether our guy was the guy that did this,'' said Browne.

Browne said the computer age has brought new security concerns because most top-secret documents now are on computers and not on paper, and communications in and out of the lab is largely by computer.

At the Energy Department's direction, the lab has tightened security on its 20,000 computers, about 3,000 of which are used for classified work.

He said additional firewalls have been established separating classified and unclassified computer systems and workers' electronic mail is being monitored.

``We've got to demonstrate to people that the lab ... can be trusted. The thing that hits me most is a feeling that the country no longer trusts Los Alamos,'' said Browne.