In some instances, agents took the open-book test together, violating rules that they take it alone. Others finished the lengthy exam unusually quickly, current and former officials said.
In Columbia, S.C., agents printed the test in advance to use as a study guide, according to a letter to the inspector general from the FBI Agents Association that summarized the investigation. The inspector general investigation also was confirmed by current and former officials who spoke on condition of anonymity because of the sensitivity of the case.
"There are similar stories for practically every office, demonstrating the pervasive confusion and miscommunication that existed," Konrad Motyka, the association's president, wrote May 13 in the letter obtained by The Associated Press.
Depending on the outcome of the investigation, agents could be disciplined or even fired.
FBI Director Robert Mueller on Wednesday acknowledged the cheating in Senate testimony, reports CBS News Radio's Bob Fuss.
Muller said he didn't know how many agents cheated on the tests, designed to make sure they understood the limits on post-9/11 powers to use surveillance and investigate people even if there is no evidence they committed a crime.
The inquiry threatens to be another black eye for the FBI as it tightens controls after years of collecting phone records and e-mails without court approval. The brewing scandal has already upended management at one of the nation's largest field offices.
The FBI had no comment on the investigation late Tuesday.
Motyka's letter urges the inspector general to focus instead on what he called the "systemic failure" of administering the test without consistent rules.
FBI agents should not be punished "because of a failure to effectively communicate the rules," he wrote.
Such testing is unusual. FBI agents are required to take online training courses to stay current on bureau policies, but pass-fail tests are rare. In 2008, however, when the FBI received more leeway than ever in conducting surveillance and opening investigations, it assured Congress that it would train and test its agents to make sure they knew the rules.
The Domestic Investigations and Operation Guidelines allowed the FBI, for the first time, to conduct surveillance for national security purposes without evidence of a crime. Agents were also allowed to consider race when opening early inquiries. For instance, the FBI could look into whether the terrorist group Lashkar-e Taiba had taken hold in a city if it had a large Pakistani-American presence.
The new rules gave agents more flexibility to identify and prevent terrorist attacks. They also raised concerns that the FBI would use its new powers to monitor religious organizations or single out certain races.
The FBI has a checkered past when it comes to conducting surveillance. From the late 1950s though the early 1970s, the bureau opened hundreds of thousands of files on Americans and domestic groups, including anti-war organizations, civil rights groups and women's movements. After the Sept. 11, 2001, terrorist attacks, the bureau collected U.S. phone and computer records without court orders.
Lawmakers and civil liberties groups were concerned that the new rules would allow racial profiling and other abuses. The FBI assured them they would not.
"We share the concern and have devoted considerable time and effort to educating our employees regarding how race and ethnicity can - and cannot - be used," FBI counsel Valerie Caproni told Congress in December 2008.
But problems with the training and testing programs surfaced quickly. Last year, Assistant Director Joseph Persichini, the head of the FBI's Washington field office that investigates congressional wrongdoing and other crime in the nation's capital, retired amid a review of test-taking in his office.
Persichini took the test alongside two of his most senior managers and one of the bureau attorneys in charge of making sure the exam was administered properly, current and former officials said. The two agents who took the test with him have been moved to headquarters while the investigation continues.
At the time, the inquiry appeared limited to the Washington field office. But investigators have broadened their inquiry to cover the entire FBI. Among other things, they are focusing on agents who took the test particularly quickly, officials said.