The audit findings prompted the "incensed" US attorney-general, Alberto Gonzales, to order the FBI to place new safeguards over its use of so-called national-security letters to secretly demand telephone, e-mail and financial records, the Post said in its Friday edition.
Tasia Scolinos, spokeswoman for the attorney-general, was quoted as telling the Post: "These past mistakes will not be tolerated."
National-security letters allow the FBI to compel the release of private information such as communications or financial records without getting authority from a judge or grand jury. Their use has grown exponentially since the attacks of September 11, 2001, the Post said.
The FBI in 2005 reported to Congress that its agents had delivered a total of 9,254 national security letters seeking e-mail, telephone or financial information on 3,501 US citizens and legal residents over the previous two years.
The congressionally mandated audit found that in 2005 alone, the FBI issued more than 19,000 national security letters amounting to 47,000 separate requests for information, the Post said.
|"[The findings are] a profoundly disturbing breach of public trust [and] somebody has a lot of explaining to do"|
Charles Schumer, member of the Senate Judiciary Committee
Critics have accused George Bush's administration of weakening civil liberties protections in its "war on terrorism".
Charles Schumer, a Democratic senator and member of the Senate Judiciary Committee that oversees the FBI, called the reported findings "a profoundly disturbing breach of public trust."
He said: "Somebody has a lot of explaining to do."
In their sampling of 293 letters, investigators found that 22 errors were possible violations of department rules and some were potential violations of law, the Post reported, citing officials with access to the audit.
The FBI identified 26 potential violations in other cases in the audit, which was limited to 77 case files in four FBI field offices, the Post said. It said officials believe the 48 known problems may be the tip of the iceberg in a "shoddy" internal oversight system, but that the problems were not deliberate.
In at least two cases cited by the newspaper, the investigators found that the FBI obtained full credit reports whereas the security letters could only be used to obtain summary information.
In other cases, telephone companies, banks or Internet providers responded with detailed personal information about customers that the letters do not permit to be released, the article said.
A justice department official who spoke on condition of anonymity told the paper that Gonzales learned of the findings three weeks ago. He "was incensed when he was told the contents of the report", the official was quoted as saying.
National-security letters have been the subject of legal battles in two federal courts because recipients were barred from telling anyone about them.
The American Civil Liberties Union sued the Bush administration over what the watchdog group described as the security letter's gag on free speech.
Last May, a federal appeals judge in New York warned that government's ability to force companies to turn over information about its customers and keep quiet about it was probably unconstitutional.