U.S. Attorney General Eric Holder received at least five memos beginning July 2010 on the botched "Fast and Furious" operation, which allowed guns bought by drug straw purchasers to "walk" into Mexico and end up in the hands of criminals, two Republican lawmakers said Thursday.

Sen. Charles Grassley and Rep. Darrell Issa said in a joint statement that Holder received several memos, including four in consecutive weeks, that described the failed strategy of the Bureau of Alcohol, Tobacco, Firearms and Explosives.

The reports were sent to Holder by Michael Walther, director of the National Drug Intelligence Center, they said.

In May, Holder told the House of Representatives' Judiciary Committee that he had learned about Fast and Furious - which was launched in 2009 by the ATF's Phoenix office and shut down late last year after guns from the program were linked to the killing of a U.S. border agent - "over the last few weeks."

But the reports sent to Holder in 2010 specifically mentioned that some intermediaries, or straw buyers, were "responsible for the purchase of 1,500 firearms that were then supplied to Mexican drug-trafficking cartels," the statement said.

"With the fairly detailed information that the attorney general read, it seems the logical question for the attorney general after reading in the memo would be 'why haven't we stopped them?'," Grassley said in a statement.

In that regard, both legislators criticized Holder for not coming clean to Congress and the American people about what he and other high-ranking Justice Department officials knew about Fast and Furious.

"The lack of candor and honesty from our nation's chief law enforcement officials in this matter is deeply disturbing," Issa said.

Justice Department officials, meanwhile, have told the media that Holder periodically is briefed on multiple investigations throughout the United States and that the reports he received on Fast and Furious in July 2010 did not explicitly state that ATF agents were letting guns "walk" into Mexico via illegal sales to known straw purchasers at U.S. gun stores.

The botched operation is the subject of separate investigations by Congress and the Justice Department and has led to a personnel shakeup at the ATF.

The idea behind Fast and Furious was to trace the weapons to powerful drug traffickers in Mexico, but once it got underway 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.

The ATF devised the sting due to criticism that its operations for years had focused on building lesser cases against gun runners rather going after the "big fish" behind the weapons purchases.

Authorities lost track of some 2,000 Fast and Furious weapons, which apparently ended up in the hands of drug traffickers, and the scandal over the botched operation sparked friction in U.S.-Mexican relations.

Mexican authorities have said that guns linked to the operation appeared at more than a hundred crime scenes in that country, while two Fast and Furious assault rifles were recovered at the location where U.S. Border Patrol agent Brian Terry was killed last December in southern Arizona.

As part of their probe into Fast and Furious, U.S. prosecutors have brought charges against a score of people, although none is suspected of drug trafficking.

Grassley also said this week that Congress should investigate another similar sting operation dubbed Operation Wide Receiver that the ATF carried out in 2006 and 2007 during former President George W. Bush's administration.

Nine people have been charged with making false statements in the purchase of firearms destined for shipment to Mexico as part of that earlier sting.

Turf battles among drug cartels and clashes between mobsters and security forces have left more than 40,000 dead in Mexico since President Felipe Calderon took office in late 2006.

Calderon says the United States is responsible in large part for the violence because of the high demand for illegal drugs there and the cross-border flow of weapons to the violent cartels.