The Center for Public Integrity is reporting that an unnamed former ABC News journalist was an FBI informant during and after the 1995 Oklahoma City bombing, passing along tips and revealing a source. We know who it is.

According to an FBI memo obtained by the Center, a nameless ABC News journalist contacted the FBI on the evening of the bombing to pass along information he had heard from a source: That the "bombing was sponsored by the Iraqi Special Services" and that two more attacks in Los Angeles and Houston were imminent.

The Center didn't name the informant, but we've learned he was Christopher Isham, who is now a vice president at CBS News and the network's Washington bureau chief.

Isham's tip was of course not true, and ABC News never reported it. But the FBI found him useful enough to open an informant file on him, and circled back a year later to ask who his or her source was. Astonishingly, Isham gave him up:

Nearly a year later, the network staffer was contacted by the FBI and agreed to divulge ABC's source for the uncorroborated claim: a former CIA officer named Vincent Cannistraro, who was on contract to the network as a consultant, who, in turn, had gotten the information from a Saudi general.

During the 1996 re-interview, the ABC employee was questioned about the "source of questioned information" and "advised that the source was VINCENT CANNISTRARO, former Counter-Terrorism Chief of the Central Intelligence Agency (CIA)," the memo stated.

That is, needless to say, a strict no-no, assuming Cannistrano provided the information off the record. Cannistraro told the Center he was "surprised that an ABC journalist had contacted the FBI and relayed the information, in part because it had not been corroborated and was just a rumor passing through Saudi circles." In the end, it didn't matter, as Cannistraro had independently gone to the FBI with the same information on the night of the bombing.

The documents don't show any more contacts between the feds and the informant, but they do show that "he or she was still being vetted for suitability as a snitch after providing 'highly accurate and reliable information in the past,'" according to the Center.

ABC News told the Center that the staffer in question no longer works for the network, and multiple knowledgeable sources have told Gawker that Isham is the rat. Isham was at ABC News from 1978 to 2007, meaning he'd been at ABC News for 17 years in 1995—matching the FBI's description of him at the time as "a senior official employed by ABC News for over 15 years." He ran the investigative unit at ABC News, putting him in regular contact with counterterrorism officials. In 1998, according to his CBS News bio, he organized the first network interview with Osama bin Laden. And his relationship with the FBI went beyond the professional: He was "close friends" with former FBI counterterrorism chief John O'Neill, according to this interview Isham gave to Frontline. (O'Neill was killed in the World Trade Center on 9/11.)

Isham, reached by cell phone, declined to comment and referred questions to a CBS News spokeswoman, who said only: "This is a matter for ABC News." (Update: Isham has released a statement. See below.)

Cannistraro did not respond to email and phone messages. Update: Cannistraro returned our call. He has no direct recollection of whom he talked to at ABC News about the Iraq rumor, so he can't confirm it was Isham. "I don't know if it was Chris or not," he says. "When I got the information, I immediately told the FBI. But I was working for ABC News as an independent contractor, so I'm sure I must have passed it on to them after. What I didn't know was that they'd be passing it on the FBI themselves."

It bears noting that it is a reporter's job to talk to sources, and since one-way conversations can get pretty boring and useless, information inevitably gets traded. Indeed, one way any good reporter would check out a tip about Iraqi involvement in Oklahoma City would be to run it by an FBI source. What's more unusual is to submit to an interview one year later, for agents in the middle of a criminal investigation, and to reveal your sources of information. That sort of cooperation is the kind of thing that can get an informant file opened on you.

Here's what CBS News had to say about the Isham affair, before it knew that Isham was involved:

The revelation, taken from a declassified FBI memo, raises questions about journalistic integrity and law enforcement ethics, as journalists are expected to keep confidential sources out of the hands of law enforcement unless there is an imminent danger to lives or the public good, neither of which appears the case.

The Oklahoma City case wasn't the only time Isham ran into the shady territory between covering law enforcement and aiding it. After he produced the bin Laden interview, according to Frontline, his pal O'Neill asked to see the unedited version. Isham demurred, citing ABC News policy against sharing unedited footage with the government. But after O'Neill persisted, Isham got the bright idea to put the whole interview online, elegantly dodging the issue.

"It's something that we often have to fight as journalists," Isham explained to Frontline, "to convince people that we are not connected and not working for a law enforcement organization or an intelligence organization."

Update: Isham released a statement earlier this evening:

The suggestion that I was an informant for the FBI is outrageous and untrue. Like every investigative reporter, my job for 25 years has been to check out information and tips from sources. In the heat of the Oklahoma City bombing, it would not be unusual for me or any journalist to run information by a source within the FBI for confirmation or to notify authorities about a pending terrorist attack. This is consistent with the policies at every news organization. But at no time did I compromise a confidential source with the FBI or anyone else. Mr. Cannistraro was not a confidential source, but rather a colleague - a paid consultant to ABC News who had already spoken to the FBI about information he had received.

[Photo of Oklahoma City via AP. Photo of Isham via CBS]