Updated

Congressional investigators probing the botched federal gun-trafficking program known as Fast and Furious are now setting their sights on the FBI.

Rep. Darrell Issa, R-Calif., chairman of the House Oversight and Government Reform Committee, and Sen. Charles Grassley, R-Iowa, the ranking member on the Judiciary Committee, sent a letter Thursday to FBI Director Robert Mueller demanding answers about the guns found at the murder scene of Border Patrol Agent Brian Terry.

Terry was gunned down last December in a remote area of the Arizona desert by a group of illegal immigrants. Two of the guns recovered at the scene were later traced to Fast and Furious, the ATF program where more than 2,000 illegally purchased guns were “allowed to walk” or were not tracked, with many crossing into Mexico.

In the letter, the lawmakers ask 16 detailed questions about the number of weapons discovered after Terry’s death. While the Justice Department has maintained that only two guns were found, the Republican lawmakers say there are inconsistencies in documents including the search warrant and the unsealed indictment.

“Out of deference to the FBI’s ongoing criminal inquiry, we have delayed asking the Bureau direct questions about the case for nearly 10 months,” they wrote. “However, Congress has a right and a duty to conduct oversight of federal law enforcement operations.

“Agencies like the FBI can wield tremendous power and influence over individual citizens. With that power and influence comes responsibility, and nothing exempts the Bureau from accountability to Congress as well as to the courts.”

They asked Mueller whether the FBI believes a third weapon killed Terry, since ballistic reports on the two Fast and Furious guns were inconclusive. They also asked the total number of suspects that Terry’s unit encountered in the desert, and how many of them are still at large.

So far, only one man, Manuel Osorio-Arellanes, has been charged with Terry’s murder. According to documents, Osorio- Arellanes told investigators that he was travelling with four other armed individuals. The letter said that it appears that the men who shot Terry may have been armed with five rifles.

The lawmakers also referred to statements made to Terry’s mother at his funeral that three weapons were recovered at the scene. Along with responses to their questions, the letter also asks for all documents and communications between nine FBI officials relating to Operation Fast and Furious. They set a deadline of November 2 for the information.

In response, the FBI said Mueller is out of town, and it's unclear if he has seen the letter.

On Monday, the Justice Department responded to Issa’s accusations about a possible third gun saying, "The FBI has made clear that reports of a third gun recovered from the perpetrators at the scene of Agent Terry's murder are false.” They also maintain that Issa’s staff was previously informed of this.

“Unfortunately, this most recent false accusation not only maligns the dedicated agents investigating the murder of Agent Terry, it mischaracterizes evidence in an ongoing case, the Just Department said in its statement.

Rep. Elijah Cummings, the top Democrat on the oversight committee, took Issa to task for sending the letter to Mueller.

"Frankly, I am shocked that Chairman Issa would continue to spin this conspiracy theory -- that the FBI is hiding a third weapon -- even after his recent allegations proved false," he said in a statement. "Rather than acknowledging this embarrassing mistake and apologizing for making false accusations about the FBI, Chairman Issa's letter is an unprecedented attack on the integrity and credibility of law enforcement that could seriously jeopardize the ongoing prosecution."

Meanwhile, Rep. Adam Schiff ,D-Calif., has come to the defense of Attorney General Eric Holder, saying the “politically motivated attacks” on him “need to come to an end.”

“They are a meritless distraction from the important work of the Department of Justice, and the many men and women who work every day to make America safer,” he said in a statement.

Schiff argued the evidence shows that Holder has been “forthright throughout” by requesting a full investigation by the inspector general once he learned of the operation’s problems. He added that Issa’s own staff was briefed on the operation in April 2010.

“Mr. Issa said nothing about the operation at the time – one might just as well call for an investigation of his office – but none is required,” he said. “Mr. Issa should understand better than most that being made aware of a program’s existence is not the same as being apprised of the operational details of a plan that has gone terribly wrong.”