Washington ? The FBI’s most damaging spy left clues over two decades, but lax security and the bureau’s reluctance to suspect one of its own helped Robert Hanssen elude detection, according to a harsh Justice Department report.

The investigation by agency Inspector General Glenn A. Fine credits the FBI for undertaking reforms since the counterespionage official’s arrest in early 2001 but warns that ongoing security flaws — such as an inability of agents to immediately know if someone reads their sensitive computer files — make the bureau vulnerable.

“We believe that what is needed at the FBI is a wholesale change in mind-set and approach to internal security,” the report said. “The defects in the FBI’s security program were the product of decades of neglect.”

Fine’s conclusions are contained in a 31-page summary of a 674-page top-secret report that included extensive interviews with 200 people, including Hanssen and his family and friends, and a review of 360,000 pages of documents. The summary was released Thursday.

The report makes 21 recommendations for change at the FBI.

Justice and FBI officials say the Hanssen case has spurred wholesale change at the FBI, including far more frequent polygraph testing, financial disclosures for employees and a central electronic system to monitor the actions of employees.

Hanssen spied for the Soviet Union and Russia between 1979 and 2001. The report shows he often was reckless in his efforts but went undetected because the bureau lacked deterrents such as intensive financial disclosure rules, frequent polygraph tests or adequate supervision.

During his 25-year FBI career, Hanssen never took a polygraph test and underwent only one financial background investigation. For his spying, Moscow paid him with two Rolex watches and $600,000 in cash and diamonds.

Besides giving away U.S. secrets, Hanssen is believed responsible for the deaths of at least three U.S. spies overseas. He pleaded guilty in May 2002 and was sentenced to life in prison.