The Fast and Furious program, run by the ATF's Phoenix office, monitored weapons purchases by suspected gun traffickers who were believed to be funneling weapons to Mexican drug cartels. Some lawmakers say ATF didn't have the means to track the guns and shouldn't have used such tactics.

An ATF spokesman declined to comment, citing ongoing investigations, including one ordered by Attorney General Eric Holder that the Justice Department's inspector general is conducting. Ricardo Alday, spokesman for the Mexican Embassy in Washington, also declined to comment.

Problems with Fast and Furious are emerging as one of the biggest controversies at the ATF since a lethal 1993 raid on the Waco, Texas, compound of a religious sect known as the Branch Davidians. Fast and Furious, which became public after ATF whistle-blowers contacted lawmakers earlier this year, is also now the subject of congressional probes.

Mexican and U.S. officials say weapons trafficked mostly from U.S. border states are fueling the cartel wars that have killed more than 40,000 people in Mexico since 2006.

The lawmakers claim the operation allowed suspected traffickers to buy more than 2,500 weapons in the U.S. and may have helped fuel the trafficking the ATF is supposed to try to prevent.

Mexican police in late April raided a home in Ciudad Juarez, across the border from El Paso, Texas. They reported finding a cache that included two dozen AK-47-style rifles, three antiaircraft machine guns, dozens of grenades and more than 26,000 rounds of ammunition.

U.S. authorities have identified at least five Romanian-made AK-47-style rifles as having been purchased in the U.S. by suspects the ATF was tracking in the Fast and Furious operation. Documents from the ATF's National Tracing Center detail the makes and serial numbers of the firearms, their recovery in Mexico, and dates in 2009 and 2010 when the ATF entered them into a database of suspect guns.

Gun traffickers often depend on a network of "straw" buyers, who are paid to buy guns in small quantities. The ATF's aim was to track purchases and build evidence to bring a case against higher-level traffickers.

Other weapons used in recent border violence, including a December shootout that killed a border patrol agent in Arizona, have also been traced to the ATF operation, according to Iowa Sen. Chuck Grassley, the top Republican on the Senate Judiciary Committee, and Rep. Darrell Issa (R., Calif.), chairman of the House Oversight Committee. The two are leading the congressional probes.

Mr. Holder told a House hearing last month the Justice Department's policy is to stop weapons from being trafficked to Mexico. "Under no circumstances should guns be allowed to be distributed in an uncontrolled manner," he said. Mr. Holder said the department issued orders to the ATF and other agencies specifically barring tactics like those alleged to have been used by the ATF.

Mr. Issa is planning hearings next week, including one expected to include the family of Brian Terry, the border agent killed in December.

The matter is growing into a political dispute, with GOP lawmakers accusing the Justice Department and the Obama administration of stonewalling their efforts to investigate. Mr. Grassley doesn't have subpoena power in the Democratic-controlled Senate, but Mr. Issa does. He contends Fast and Furious must have had approval from top Justice officials in Washington.

The Justice Department said last month that the Fast and Furious operation was first approved by the U.S. attorney's office in Arizona and the ATF's Phoenix field office, and later by a multiagency organized-crime task force.

Tracy Schmaler, a Justice Department spokeswoman, said: "The attorney general takes the allegations that have been raised seriously, which is why he has asked the inspector general to investigate and made clear to everyone in the department that under no circumstances should guns be allowed to cross the border."

In a 2010 audit, the Justice Department inspector general criticized the ATF for pursuing too many small-buyer cases and not using its resources to find major gun traffickers. People familiar with the ATF operation say Fast and Furious was conceived in part as a response to such criticism, and aimed to use straw buyers to implicate bigger trafficking bosses.

Gun-rights groups, which dispute that the U.S. is a major source of firearms trafficked to Mexico, have criticized ATF attempts to regulate gun purchases. At the same time, the Obama administration has been under pressure from big-city mayors and others who favor tighter restrictions.

The ATF has been without a Senate-approved director since 2006, with both the Bush and Obama administration unable to win approval for nominees over the objections from gun-rights groups and lawmakers.