The investigators have asked the FBI and the Drug Enforcement Administration for details about the alleged informants, as well as why agents at the Bureau of Alcohol, Tobacco, Firearms and Explosives, which ran the Fast and Furious operation, were not told about them.

The development raises further doubts about the now-shuttered program, which was created in November 2009 in an effort to track guns across the border and unravel the cartels' gun-smuggling networks.

The gun tracing largely failed, however, and hundreds of weapons purchased in U.S. shops later were found at crime scenes in Mexico.

The scandal has angered Mexican officials and some members of Congress. Investigators say nearly 2,500 guns were allowed to flow illegally into Mexico under the ATF program, fueling the drug violence ravaging that country and leading to the shooting death of a U.S. border agent.

In a letter to FBI Director Robert Mueller, the investigators asked why U.S. taxpayers' money apparently was paid to Mexican cartel members who have terrorized the border region for years in their efforts to smuggle drugs into this country, and to ship U.S. firearms into Mexico.

"We have learned of the possible involvement of paid FBI informants in Operation Fast and Furious," wrote Rep. Darrel Issa, R-Calif., chairman of the House Committee on Oversight and Government Reform, and Sen. Charles Grassley, of Iowa, the top Republican on the Senate Judiciary Committee.

The two have been the leading congressional critics of the program.

"At least one individual who is allegedly an FBI informant might have been in communication with, and was perhaps even conspiring with, at least one suspect whom ATF was monitoring," they wrote.

FBI, DEA kept silent

The FBI and DEA did not tell the ATF about the alleged informants. The ATF and congressional investigators learned later that those agencies apparently were paying cartel members whom the ATF wanted to arrest.

"Operation Fast and Furious was conceived to get some of the bigger fish down there," said one official close to the congressional investigation, who asked not to be identified because the probe is ongoing. "Then it turned out that some of the ones they were zeroing in on actually, mostly likely were paid informants."

The official said at least half-a-dozen cartel figures were being paid by one U.S. law enforcement agency while they were being targeted by another.

"We are learning more about them, and there are six so far that we know about. It's multiples, for sure," the official said.

The official declined to identify the six, but said the individual cited in the letter to the FBI operated along the Texas border with Mexico.

"This guy could die if it got out who he is," the official said. "He'd be killed."

AK-47s at death scene

The FBI and DEA referred queries Saturday to the Department of Justice. Officials there said they cannot talk about paid informants but are attempting to answer the questions from Issa and Grassley.

They noted that they already have provided thousands of pages of documents to Congress and have answered questions in sworn depositions and open hearings.

"We have been cooperating, and we will continue to cooperate," said an Obama administration official. "We want to learn as much as we can about Fast and Furious, just as they do."

The ATF ran the Fast and Furious operation out of its field office in Phoenix until early this year. The idea was to monitor illegal purchases in federally licensed U.S. gun shops, but allow the illicit buyers to walk away with the weapons, giving ATF agents the opportunity to learn how the guns are smuggled across the border.

But many of their gun-tracing efforts failed, and hundreds of firearms purchased under the program were used to commit crimes in Mexico.

In addition, two AK-47s purchased during the operation were found at the scene last December where U.S. Border Patrol Agent Brian Terry was shot to death near Tucson, allegedly by a Sinaloa cartel member.

A second U.S. immigration agent, Jaime Zapata, was slain in a cartel ambush in February in Mexico, and investigators now are trying to determine whether Fast and Furious weapons were used to kill him as well.

Questions about Zapata

In the letter to Mueller, the senators asked if Zapata was armed when he was killed and "if not, why not?" They also asked if his vehicle was bulletproof, and if so, "how was he killed inside of the vehicle?"

They also asked how many cartel members were paid, if any of the informants had been deported from the United States, and if FBI personnel in Arizona knew of the informants.

The Republican leaders asked to see all "emails, documents, memoranda, briefing papers, and handwritten notes" from and to FBI and DEA field supervisors and agents in Phoenix, Tucson, Nogales and Yuma in Arizona, and in El Paso.

They are seeking the same material from the FBI case agent investigating Terry's death.

Zapata's family also is seeking answers.

Relatives of the slain immigration agent sent a letter June 14 to the U.S. attorney's office in South Texas, the top FBI official in San Antonio, and several U.S. immigration officials, "requesting information about the specific circumstances of Jaime Zapata's death."