The Justice Department Inspector General’s internal audit, released Wednesday, harshly criticized how the Federal Bureau of Investigation’s Communications Analysis Unit — a counterterrorism section founded after 9/11 — relied on so-called “exigent” letters to get carriers to turn over phone records immediately. The letters were a hangover from the investigation into the 9/11 attacks in New York and promised telecoms, falsely, that subpoenas would follow shortly.

“The FBI’s use of exigent letters and other informal requests for telephone toll billing records circumvented, and in many cases violated, the requirements of the Electronic Communications Protection Act statute,” according to the report, which was referencing a leading federal wiretap law.

“We found that a distinct lack of oversight and scrutiny by CAU managers, counterterrorism officials and FBI Office of General Counsel attorneys enabled the improper practice of obtaining ECPA-protected telephone records with the promise of future legal process to expand and proceed virtually unchecked for over 4 years,” the report found.

But in a surprise buried at the end of the 289-page report, the inspector general also reveals that the Obama administration issued a secret rule almost two weeks ago saying it was legal for the FBI to have skirted federal privacy protections.

It all started after 9/11 as FBI agents hunting terrorists began relying heavily on call toll records — essentially a person’s phone bills — in a frantic effort to find terror plots before they were carried out. The agency even gave AT&T, Verizon and MCI multimillion dollar contracts to keep phone records longer and to answer FBI requests faster.

The companies then set up remote terminals inside the FBI’s offices, staffing them with telecom employees who quickly became friendly with the agents requesting phone records. The telecom employees had FBI e-mail addresses, access to shared drives and invitations to happy hours, according to the report.

The telecom employees were supposed to be responding to National Security Letters, which are essentially FBI-issued subpoenas. But those Patriot Act powers say the target must be part of an open investigation and that a supervisor has to approve it. While they require some paperwork, FBI agents have been issuing about 40,000 such NSLs a year.

But an AT&T employee provided the unit with a way around some of those requirements. The employee introduced them to “exigent letters.” Those letters, first used immediately following 9/11, asked for information by saying that the request was an emergency and that prosecutors were preparing a grand jury subpoena. The letter falsely promised that the subpoena, which gives the telecoms legal immunity, would be delivered later, the report said.

What’s more, the report noted that the cozy relationship between the bureau and the telecoms made it hard to differentiate between the FBI and the nation’s phone companies.

“The FBI’s use of exigent letters became so casual, routine and unsupervised that employees of all three communication service providers told us that they — the company employees– sometimes generated the exigent letters for CAU personnel to sign and return,” the inspector general reported.

In fact, one AT&T employee even created a short cut on his desktop to a form letter that he could print out for a requesting FBI agent to sign.

Even that became too much. Agents would request “sneak peeks,” where they’d ask if it was worth their time to file a request on a given phone number, the inspector general noted. The telecom agents complied. Soon it graduated to numbers on Post-it notes, in e-mails or just oral requests.

There’s no telling how many of those there were, but the audit estimates more than 3,500 off-the-book requests from 2003 to 2007.

AT&T also offered to look up calling information for those connected to the original target in what is known as a “community of interest,” according to the report. That term refers to the social networks learned by looking at who is calling whom — a leap in degrees of separation from your initial target.

In its statement on the report, the FBI excused itself for its failure to follow the law.

“The OIG report finds no intentional attempts to obtain records that counterterrorism personnel knew they were not legally entitled to obtain,” said Michael P. Kortan, the FBI’s assistant director for public affairs.

“No FBI employee obtained telephone records for reasons other than a legitimate investigative interest. FBI employees involved in this matter obtained the telephone records at issue to perform their critical mission to prevent a terrorist attack or otherwise to support a counterterrorism investigation.”

(The report refers to the three telecoms involved as Company A, B, and C, but previous statements by FBI officials have helped sort out the details.)

An AT&T spokesman declined comment on the report. Verizon, which has acquired MCI, did not immediately respond to a request for comment.

AT&T took a particular interest in using its “community of interest” technology to become analysts for the agents, the report said.

For instance, according to the report, an AT&T analyst looked at one informal FBI request and said the calling patterns on four numbers were “very interesting,” and “we strongly suggest” the bureau examines other, associated phone numbers.

The telecoms also violated wiretapping law by instituting more than 100 so-called “hot watches” that would alert FBI agents if a targeted number made or received a call, the report said.

What’s more, FBI agents also used the exigent letters in a leak investigation to get records on more than 1,500 calls by Washington Post and New York Times reporters.

Many FBI higher-ups denied knowledge of the loose and illegal practices the inspector general cited, the report said.

But was there any connection between these exigent letters and the National Security Agency’s warrantless wiretapping program?

The report — and other memos — strongly suggest many of the FBI’s leads came from the NSA listening in on Americans’ overseas calls and e-mails.

Many times the exigent letter or verbal request for telephone records turned up areas worth opening a more detailed investigation. In some cases, they showed the agent that a full-blown wiretap was necessary, the report said. But permission for listening to a suspected terrorist’s phone calls has to come from the nations’ secret spy court, the Foreign Intelligence Surveillance Court.

But how do you get a search warrant when the evidence you have to support it came from an illegal request? How do you explain to a judge you got your probable cause illegally?

You don’t. You “make false statements” in your affidavit. That’s what agents did in order to use the poisoned fruit of these secret searches, according to the report.

Years later — after the inspector general’s earlier reports on the exigent letters, the National Security Division investigated a few of the affidavits, found the “false statements” and told the FISA court about the lies.

But the Justice Department said in the report that false statements don’t matter — the wiretap orders from the FISA court remain usable even if evidence is acquired unlawfully, the report noted.

The FBI kicked the telecoms out of its offices in 2007, saying the bureau and telecom employees had become too comfortable working together.

The Obama administration retroactively legalized the entire fiasco through a secret ruling from the Office of Legal Counsel nearly two weeks ago.

That’s the same office from which John Yoo blessed President George W. Bush’s torture techniques and warrantless wiretapping of Americans’ communications that crossed the border.

In the report’s final and heavily censored section, it discloses that the Office of Legal Counsel issued an opinion that it was legal for the FBI to obtain Americans’ phone records in the same manner that was harshly criticized by the inspector general’s report.

The inspector general also warned that the Office of Legal Counsel’s ruling needs to be considered by Congress.