In the upside-down world of counterintelligence investigations, innocent habits became incriminating actions, leaving the suspect no escape from his nightmare.

The real O'Neill knew going in that Hanssen was the subject of a counterintelligence investigation.

Our counterintelligence investigations also help combat international terrorist threats, including those involving weapons of mass destruction and attacks on critical infrastructures.

"We've shared this as appropriate with law-enforcement authorities in the United States, and there are a number of different counterintelligence investigations proceeding from this."

When the F.B.I. learned of these matters, it opened a foreign counterintelligence investigation.

Then FBI briefed several counterintelligence investigations that might be related.

Mr. Nicholson fell under heavy counterintelligence investigation early in the year.

It is still unclear what the connection might be between that raid and the continuing counterintelligence investigation of the possible leaks of secrets to Iran.

The officials said that the arrest followed a counterintelligence investigation and that it would be announced on Thursday.

They said counterintelligence investigations were almost never solved quickly, but often required years of painstaking work.