A couple of Capitol Hill lawmakers are continuing their push for details of Operation Fast and Furious, a Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) gun running operation that purposely trafficked thousands of assault weapons across the U.S. border with Mexico. Guns traced back to the operation have turned up at crime scenes in Mexico and near a murdered Border Patrol Agent in Arizona.

In a letter to the agencies' heads, Sen. Chuck Grassley, R-Iowa, and House Oversight Committee Chairman Darrell Issa, R-Calif., call for information from the FBI and the Drug Enforcement Agency (DEA), including interviews with agents, lists of informants, emails and even handwritten notes about their roles in the operation. The letter says neither the FBI nor the DEA has responded to earlier requests for additional information in the case.

"On March 15, 2011, Senator Grassley sent you a letter requesting a briefing to gain a better understanding of the Drug Enforcement Administration's (DEA) involvement in Operation Fast and Furious," the letter to DEA administrator Michele Leonhart reads. "Nearly four months later, your agency has yet to respond directly."

The questions for the FBI are even more pointed and include requests for details of the investigation into the murder of Immigrations and Customs Enforcement (ICE) officer Jamie Zapata. Zapata was unarmed and gunned down, along with a fellow agent who was wounded, in northern Mexico in February.

"To help us better understand the role of the FBI in this multi-agency...case, we request that you produce communications relating to Operation Fast and Furious by FBI personnel based in Phoenix, Arizona, Tucson, Arizona, and El Paso, Texas..." the letter to FBI director Robert Mueller read.

Grassley and Issa have been spearheading a congressional investigation into Operation Fast and Furious in the months since it came to light.