Updated

Just hours after the death of Border Patrol agent Brian Terry, federal officials tried to cover up evidence that the gun that killed Terry was one the government intentionally helped sell to the Mexican cartels in a weapons trafficking program known as Operation Fast and Furious.

The revelation comes just days after a huge shake-up of government officials who oversaw the failed anti-gun trafficking program and Congress renewed its demand for more answers.

Also late Thursday, Sen. Charles Grassley's office revealed that 31 more Fast and Furious guns have been found at 12 violent crime scenes in Mexico.

In an internal email the day after Terry's murder, Assistant U.S. Attorneys Emory Hurley and then-U.S. Attorney Dennis Burke decided not to disclose the connection, saying "this way we do not divulge our current case (Fast and Furious) or the Border Patrol shooting case."

Grassley, R-Iowa, and Rep. Darrell Issa, R-Calif., said Thursday they are expanding their investigation into the scandal. In a strongly worded letter to Anne Scheel, the new U.S. attorney for Arizona, the chairman of the House Oversight and Government Reform Committee requested interviews, emails, memos and even hand-written notes from members of the U.S. attorney's office that played key roles in the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) program.

“The level of involvement of the United States Attorney’s Office ... in the genesis and implementation of this case is striking,” wrote Issa and Grassley.

The two claim witnesses have told them that recently reassigned Hurley may have also prevented ATF agents from doing their jobs.

“Many ATF agents working on Operation Fast and Furious were under the impression that even some of the most basic law enforcement techniques typically used to interdict weapons required the explicit approval of your office, specifically that from Hurley,” the letter states.

Issa and Grassley said they want to speak with Hurley and Assistant U.S. Attorney Michael Morrissey, along with Patrick Cunningham, chief of the office’s Criminal Division.

Not only do congressional investigators want to "make sense" of details of the operation that allowed more than 2,000 guns to "walk" and later turn up at crime scenes on both sides of the U.S.-Mexico border, but they want to known why Hurley -- who knew almost immediately the guns found at Terry's crime scene belonged to Fast and Furious -- tried to "prevent the connection from being disclosed."

Grassley also raised concerns about the latest data on Fast and Furious guns found at crime scenes in Mexico.

“The Justice Department has been less than forthcoming since day one, so the revisions here are hardly surprising, and the numbers will likely rise until the more than 1,000 guns that were allowed to fall into the hands of bad guys are recovered -- most likely years down the road," Grassley said in a statement released Thursday.

"What we’re still waiting for are the answers to the other questions the Attorney General failed to answer per our agreement. The cooperation of the Attorney General and his staff is needed if we’re ever going to get to the bottom of this disastrous policy and help the ATF and the department move forward.”