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A third gun linked to "Operation Fast and Furious" was found at the murder scene of Border Patrol Agent Brian Terry, new documents obtained exclusively by Fox News suggest, contradicting earlier assertions by federal agencies that police found only two weapons tied to the federal government's now infamous gun interdiction scandal.

Sources say emails support their contention that the FBI concealed evidence to protect a confidential informant. Sources close to the Terry case say the FBI informant works inside a major Mexican cartel and provided the money to obtain the weapons used to kill Terry.

Unlike the two AK-style assault weapons found at the scene, the third weapon could more easily be linked to the informant. To prevent that from happening, sources say, the third gun "disappeared."

In addition to the emails obtained by Fox News, an audio recording from a Bureau of Alcohol, Tobacco, Firearms and Explosives agent investigating the Terry case seems to confirm the existence of a third weapon. In that conversation, the agent refers to an "SKS assault rifle out of Texas" found at the Terry murder scene south of Tucson.

The FBI refused to answer a detailed set of questions submitted to officials by Fox News. Instead, agency spokesman Paul Bresson said, "The Brian Terry investigation is still ongoing so I cannot comment." Bresson referred Fox News to court records that only identify the two possible murder weapons.

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However, in the hours after Terry was killed on Dec. 14, 2010, several emails written to top ATF officials suggest otherwise.

In one, an intelligence analyst writes that by 7:45 p.m. -- about 21 hours after the shooting -- she had successfully traced two weapons at the scene, and is now "researching the trace status of firearms recovered earlier today by the FBI."

In another email, deputy ATF-Phoenix director George Gillett asks: "Are those two (AK-47s) in addition to the gun already recovered this morning?"

The two AK-type assault rifles were purchased by Jaime Avila from the Lone Wolf Trading Co. outside of Phoenix on Jan. 16, 2010. Avila was recruited by his roommate Uriel Patino. Patino, according to sources, received $70,000 in "seed money" from the FBI informant late in 2009 to buy guns for the cartel.

According to a memo from Assistant U.S. Attorney Emory Hurley, who oversaw the operation, Avila began purchasing firearms in November 2009, shortly after Patino, who ultimately purchased more than 600 guns and became the largest buyer of guns in Operation Fast and Furious.

Months ago, congressional investigators developed information that both the FBI and DEA not only knew about the failed gun operation, but that they may be complicit in it. House Government Reform and Oversight Committee Chairman Darrell Issa, R-Calif., and Sen. Charles Grassley, R-Iowa, fired off letters in July requesting specific details from FBI director Robert Mueller and Drug Enforcement Administration chief Michele Leonhart.

"In recent weeks, we have learned of the possible involvement of paid FBI informants in Operation Fast and Furious," Issa and Grassley wrote to Mueller. "Specifically, at least one individual who is allegedly an FBI informant might have been in communication with, and was perhaps even conspiring with, at least one suspect whom ATF was monitoring."

Sources say the FBI is using the informants in a national security investigation. The men were allegedly debriefed by the FBI at a safe house in New Mexico last year.

Sources say the informants previously worked for the DEA and U.S. Marshall's Office but their contracts were terminated because the men were "stone-cold killers." The FBI however stopped their scheduled deportation because their high ranks within the cartel were useful.

In their July letter, Issa and Grassley asked Mueller if any of those informants were ever deported by the DEA or any other law enforcement entity and how they were repatriated.

Asked about the content of the emails, a former federal prosecutor who viewed them expressed shock.

"I have never seen anything like this. I can see the FBI may have an informant involved but I can't see them tampering with evidence. If this is all accurate, I'm stunned," the former prosecutor said.

“This information confirms what our sources were saying all along -- that the FBI was covering up the true circumstances of the murder of Brian Terry," added Mike Vanderboegh, an authority on the Fast and Furious investigation who runs a whistleblower website called Sipsey Street.

"It also confirms that the FBI was at least as culpable, and perhaps more culpable, than the ATF in the (Fast and Furious) scandal, and that there was some guiding hand above both these agencies (and the other agencies involved) coordinating the larger operation," Vanderboegh said.

Asked about the new evidence, Terry family attorney Pat McGroder said, "The family wants answers. They'd like to put this to rest and put closure to exactly what happened to Brian."