The FBI has offered a $1 million reward for information leading to the arrest of four fugitives charged in the killing of a border agent. U.S. Attorney Laura E. Duffy is flanked by wanted posters as she speaks at a news briefing to announce the indictment of the five suspects. (Ross D. Franklin/AP)

The FBI offered a $1 million reward Monday for information leading to the arrest of four fugitives charged in the killing of a border agent whose death is at the center of a Washington showdown over a botched gun-tracking operation.

The announcement came as the Justice Department unsealed an indictment charging the four men — along with a suspect who is in custody — in the killing of the agent, Brian Terry, who was shot while on patrol in an Arizona canyon in December 2010.

The indictment did not mention “Fast and Furious,” the controversial anti-gun-trafficking operation run by the Phoenix office of the Bureau of Alcohol, Tobacco, Firearms and Explosives. But it was Terry’s death that led to a congressional investigation of the operation and, more recently, a vote in the House to hold Attorney General Eric H. Holder Jr. in contempt of Congress.

During the two-year operation, ATF agents watched as hundreds of weapons were purchased by gun-trafficking suspects.

Some agents testified that they were ordered to let the guns “walk” so the agency could trace the weapons to a firearms-trafficking ring. Several of their supervisors have said that they never allowed gun-walking but were told by the U.S. attorney’s office in Phoenix that they did not have enough evidence to seize the guns.

About 2,000 guns linked to the suspects hit the streets in the United States and Mexico. Two guns tied to the operation were found at the scene of Terry’s death.

In the indictment unsealed Monday and handed up in Tucson, the five charged were identified as Manuel Osorio-Arellanes, Jesus Rosario Favela-Astorga, Ivan Soto-Barraza, Heraclio Osorio-Arellanes and Lionel Portiollo-Meza. Osorio-Arellanes was arrested the night of the shooting, but the other four remain at large.

Authorities say the suspects entered the United States illegally from Mexico for the purpose of robbing drug traffickers.

The bullet that killed Terry, 40, was so damaged that the FBI was unable to definitively link either of the two firearms found at the scene to his death, according to a law enforcement official involved in the investigation.

But lawmakers tied Terry’s death to the botched Phoenix gun operation after it was discovered that the serial numbers on the semiautomatic rifles matched guns bought by a suspect charged in the Fast and Furious case, Jaime Avila.

Law enforcement officials say that ATF never actually “watched” those particular guns being bought and could never have seized them. They say that an employee of the store where those firearms were bought did not notify ATF until after the guns were purchased and were in the trafficking pipeline.

On Jan. 16, Avila legally bought three guns at Lone Wolf Trading Company in Glendale, a suburb of Phoenix. After the purchase, an employee who was suspicious of the sale faxed the paperwork to ATF. But because it was the Martin Luther King Jr. holiday weekend, the ATF agents did not receive the fax until Jan. 19, according to several law enforcement officials and a recent article in Fortune magazine.

By that time, because it had been three days since the rifles were bought, ATF officials say that they never saw the guns or had the opportunity to seize them. But they logged the serial number from the rifles into a “suspect gun data base.”

“You tell me how the ATF caused the death of Brian Terry under any rational view of the uncontroverted facts,” said Paul Pelletier, an attorney for William D. Newell, the ATF special agent in charge of the Phoenix office who oversaw Fast and Furious.

But Jason Foster, the chief investigator for Sen. Charles E. Grassley (R-Iowa), said that Avila, the gun buyer, had for months been suspected by ATF of trafficking and that agents could have questioned him or searched his residence for the guns after he bought them.

The controversy over the case has led to an 18-month-old congressional investigation. Last month, the House voted to cite Holder in contempt of Congress for refusing to provide certain documents about the case.

On Monday, Rep. Darrell Issa (R-Calif.), who led the congressional investigation, applauded the unsealing of the indictment but “condemned the timing” and suggested that the move was politically motivated.