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Answering a knock at his front door, FBI special agent Earl Edwin Pitts was surprised to see an old acquaintance, a Russian he knew years earlier in New York City.

“There is a guest visiting me. He wanted to see you. He’s in my car. He’s from Moscow,” the man said.

Rather than inviting them in, Pitts quickly arranged for a rendezvous at a nearby Civil War site and left home in a “panic” that worried his wife, Mary.

An hour later, Pitts met his unexpected visitors at the Chancellorsville Battlefield Visitor Center in Virginia, where they asked him to provide FBI secrets for Russia and gave him an envelope with $15,000 in used $100 bills.

“I’ll do what I can,” Pitts assured them, putting the envelope in his pants pocket.

With that began a 16-month FBI “sting” operation that climaxed with Pitts’ arrest Wednesday at the FBI Academy in Quantico, Va., on charges that he spied for Moscow as far back as the late 1980s.

In court documents, the government sets out a spy-novel-like tale of clandestine meetings, cash-stuffed envelopes and “dead drop” deliveries of purloined FBI documents, including personal and medical information about fellow FBI agents and even Pitts’ own FBI identification card.

A week ago, just days before his arrest, Pitts reportedly apologized in a note to his “Russian” handlers for the poor quality of his most recent deliveries but added, “I have provided you with everything I was aware of.”

Pitts, a veteran FBI counterintelligence agent, apparently never realized he was the target of a spy investigation, that his old Russian acquaintance was now an FBI informant, and that his new “Russian” contacts were actually FBI counterintelligence agents.

In fact, he brazenly continued to sell secrets despite finding a surveillance camera hidden in the ceiling of his FBI office and despite his wife’s revelation that she had reported her suspicions to his FBI superiors.

Pitts is to appear for an arraignment hearing Friday in federal court in Alexandria, Va., where he is expected to plead not guilty.

The FBI has begun a damage assessment of Pitts’ activities between 1987 and 1992, the period when he allegedly sold secrets to the Russians for at least $224,000. During that period, officials said, Pitts had access to highly classified operations, including efforts to recruit Russian spies, to identify and monitor Russian intelligence agents in the U.S., and to debrief defectors.

FBI Director Louis Freeh said Thursday that he wouldn’t “underestimate the seriousness of the damage” but a full review was put off to avoid jeopardizing the secrecy of the undercover investigation.

To avoid tipping anyone off, the FBI agents pretending to be Russian agents operated in complete secrecy, even within the bureau. Freeh personally supervised the undercover operation to assure secrecy, and agents working on the case were subject to polygraphs.

“We were not able to do what we are doing now, which is a comprehensive review and interview of many of the people that he had worked with, looking at the files and the access which he had during the period beginning in 1987 up to the present,” Freeh told a Senate Judiciary subcommittee Thursday.

The espionage activities by Pitts, 43, an attorney and 13-year veteran who formerly monitored Soviet spies in New York, allegedly occurred at the same time CIA turncoat Aldrich Ames also was spying for Moscow.

Together, the two could have provided the Russians with a comprehensive look at U.S. intelligence. Ames knew, and exposed, U.S. spy operations in the Soviet Union and Pitts supervised FBI efforts in New York to detect Russian spying in the U.S.

Ames, however, was responsible for the deaths of at least 10 Russians and East Europeans who worked as Western agents. Pitts’ espionage efforts apparently caused no deaths, Freeh said.

Observers Thursday gave the FBI high marks for the operation, which cost upwards of $1 million.

“It shows the FBI can investigate itself thoroughly, cautiously and do a good job,” said Ronald Kessler, author of the books, “The FBI” and “Inside the CIA.”

“In this case, they were able to keep it secret from the rest of the bureau. They had separate reporting channels, polygraphed every agent working on the case and used unprecedented methods to maintain secrecy. It’s a sign the FBI can do a superb job when it wants to.”

Freeh acknowledged the delay in detecting Pitts but said the bureau has taken steps to bolster its detection efforts, including using polygraphs and reassigning more than 70 agents to internal security.

“With respect to this particular case, we have identified areas where we could’ve done and are doing a better job,” the director said.

The investigation itself almost collapsed just weeks after it began, when Pitts discovered the surveillance camera hidden in his FBI Academy office. Earlier, the camera watched him count his $15,000 payoff, methodically sorting the money into envelopes holding 10 bills each and then hiding them all in a large manila envelope behind a ceiling panel in his office, according to the government.

But before Pitts could closely examine the camera, agents attached a tag indicating it belonged to the FBI office that monitors employee attendance and hours, seemingly just routine.

Similarly, FBI officials pretended to accept the story Pitts offered after his wife’s alarm about the foreign visitor. He reportedly told FBI supervisor Tom Carter that the man at the door was a former Russian informant seeking legal advice, whom he had taken to the local Wal-Mart because of the man’s drunken condition.

In the weeks that followed, the government contends Pitts led a secret life that involved delivering FBI materials to a designated “dead drop” location just outside Washington in Fairfax County, Va., for pickup by the “Russians” and retrieving envelopes of cash from a nearby Mailboxes Etc. outlet.

The government said his deliveries included a classified manual and video on FBI counterintelligence techniques, employee files, the handset of a secure telephone system, and even a key and his identification badge to enable a Russian agent to enter the FBI Academy.

At various times, Pitts allegedly asked his “Russian” handlers if they could give him some of the more than $100,000 accumulated in a “reserve account” during his earlier period of spying for Moscow.

He said he wanted a “steady stream of payments” because big cash payments are so “difficult to hide.”