Two Republican lawmakers accused the Justice Department of obstructing the congressional probe of "Operation Fast and Furious," a botched initiative that allowed more than 2,000 guns to be smuggled into Mexico.

Among the materials being withheld is a document one senior official described as "a smoking gun," Rep. Darell Issa of California and Iowa Sen. Charles Grassley said in a letter to U.S. Attorney General Eric Holder.

The lawmakers requested that more documents be released to clear up the origin and cause of the failed operation of the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The idea behind allowing guns to be sold and smuggled into Mexico was to track them to their end-users and build criminal cases against the Mexican drug cartels, who are said to obtain much of their arsenals from gun dealers in U.S. border states.

Once Fast and Furious got under way, however, ATF agents realized they had no dependable way to keep track of the guns, which eventually began appearing at crime scenes on both sides of the border.

Grassley and Issa also sent letters to FBI Director Robert Mueller and Drug Enforcement Administration head Michelle Leonhart to find out what role those agencies played in the operation.

In their missive to Holder, the legislators said Acting ATF Director Kenneth E. Melson told them of the existence of "a smoking gun" document pertaining to Fast and Furious.

The lawmakers wrote to Holder after Melson told them, in a private session, that the Department of Justice had refused him permission to testify.

The letter includes part of the extensive statement Melson gave about what he called Department of Justice efforts to hide information and hundreds of documents related to Fast and Furious.

A partial transcript of Melson's testimony to congressional investigators deals with one case in which, "interdiction could have occurred, and probably should have occurred, but did not occur."

Melson implicated other federal agencies in the operation, including the FBI and DEA, and complained that the Department of Justice is trying to protect its high-level political appointees.

The Department of Justice must hand over all the requested documents to understand what went wrong in Operation Fast and Furious.

Among other issues, the lawmakers want to know if some of the targets of the operation - those who supplied high-caliber weapons to Mexican drug cartels - could have been paid informers of the FBI.

One of the weapons involved in the ATF operation turned out to have been used in the Feb. 15 killing of U.S. immigration agent Jaime Zapata in the Mexican state of San Luis Potosi, while another Fast and Furious gun was used to kill U.S. Border Patrol agent Brian Terry.

Congressional investigators say that besides failing to strike any of the drug cartels, Fast and Furious actually increased the level of violence in Mexico, where more than 40,000 people have died in drug-war mayhem since December 2006.