Did FBI help conceal evidence that third Gunwalker gun was at the scene where border agent was killed?

You have multiple government agencies within the Justice Department involved in this scandal and Holder wants to argue that no political appointees outside the BATFE knew what was going on? From Fox News:

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. . . .