So the Hanssen case provides fresh insight into the dilemma facing American counterintelligence experts each time they catch a spy.

Mr. Hanssen, a 25-year F.B.I. veteran and counterintelligence expert, was arrested on Sunday in a Virginia park.

But the Government's counterintelligence experts had concerns about security and counterintelligence procedures at the laboratories long before the latest case.

The story of Robert Philip Hanssen, the counterintelligence expert for the F.B.I. who was also a spy for Moscow.

Mr. Gorbachev's remark was considered highly unusual by counterintelligence experts, who noted that in no case in recent memory had the Soviet Union offered a similar reassurance.

Mr. Hanssen, who pleaded guilty to spying for Moscow last month, is being debriefed by counterintelligence experts.

Mr. Hanssen, a 25-year F.B.I. veteran and counterintelligence expert, was arrested on Feb. 18.

Mr. Hanssen, a 25-year employee and a counterintelligence expert, was arrested on Feb. 18 and later charged with spying for Russia.

The government charges that Mr. Hanssen, a 25-year F.B.I. employee and counterintelligence expert, spied for Moscow for more than 15 years.

Although American counterintelligence experts were supposed to oversee construction, security was obviously lax.