PHOENIX — A Mexican man pleaded guilty Tuesday in the killing of a US Border Patrol agent whose death revealed the botched ‘‘Fast and Furious’’ gun-smuggling operation, marking the biggest conviction to date in a case that embarrassed the federal government and prompted a series of congressional investigations.
In his guilty plea, Manuel Osorio-Arellanes admitted that he was part of a rip-off crew that sneaked into the United States from Mexico about a week before the death of Agent Brian Terry.
They stashed guns and food supplies on the US side of the border and killed Terry as they searched for marijuana smugglers to rob.
Authorities haven’t said which member of the rip-off crew was believed to have fired the fatal shot at Terry on Dec. 14, 2010.
Of the five men charged in Terry’s killing, only two are in custody, while three others remain fugitives.
Prosecutors agreed not to seek the death penalty against Osorio-Arellanes, who could face life in prison for the first-degree murder conviction.
Terry’s death lifted the veil on the bungled federal government’s gun-smuggling investigation that was later the subject of congressional probes.
Federal authorities have faced heavy criticism for allowing suspected straw gun buyers to walk away from shops with weapons, rather than arrest the suspects and seize the guns there.
Two rifles bought by a gun-smuggling ring that was being monitored through ‘‘Fast and Furious’’ were found at the scene.
But authorities have declined to say whether the murder weapon in Terry’s death was linked to a purchase from the operation.
Terry and three other agents came under attack in a canyon north of the Arizona border city of Nogales by
Osorio-Arellanes and four other men, investigators have said.
Osorio-Arellanes, of El Fuerte in the Mexican state of Sinaloa, was shot during the gunfight and has been in custody since the night of the shooting.
Osorio-Arellanes told investigators he raised his weapon toward the agents during the shootout but didn’t open fire, the FBI said.
Sentencing has been set for Jan. 11 before US District Judge David Bury. Osorio-Arellanes’ lawyer, Clay Hernandez, declined through an assistant to comment on Tuesday on the plea.
Prosecutors agreed not to seekthe death penalty against Manuel Osorio-Arellanes, who could face lifein prison for the murder conviction.
‘‘Today’s plea is an important step in seeking justice on behalf of Agent Terry,’’ Laura Duffy, the top federal prosecutor in San Diego whose office is handling the case, said in a statement.
Operation Fast and Furious was launched in 2009 to catch trafficking kingpins, but federal agents lost track of about 1,400 of the more than 2,000 weapons — including AK-47s and other high-powered assault rifles.
Some of the guns purchased illegally with the government’s knowledge were later found at crime scenes in Mexico and the United States.
Critics have hammered federal authorities for allowing informants to walk away from Phoenix-area gun shops with weapons.
The two guns found at the scene of the Terry shooting were purchased by a straw buyer for a smuggling ring suspected of purchasing guns for the brutal Sinaloa cartel, according to investigators.
Jaime Avila, 25, has admitted in court to buying the two guns and has pleaded guilty to gun charges in a smuggling case that is separate from the prosecution into Terry’s death.
Avila, who is not charged in Terry’s death, faces up to 10 years in prison when he’s sentenced on Dec. 12.
Of the four other men charged in Terry’s death, one is in custody, while three others remain fugitives.
Authorities have offered a $1 million reward for information leading to their capture.