LOS ANGELES — A Mexican pleaded guilty over the killing of a US border agent that threw a spotlight on the United States' controversial "Fast and Furious" gun-tracking operation, prosecutors said.

In a plea deal, Manuel Osorio-Arellanes admitted to first degree murder in court in Tucson, Arizona in connection with the death of agent Brian Terry, shot dead in December 2010 on the US-Mexico border.

"Agent Terry was killed in the line of duty courageously safeguarding our border," US Attorney Laura Duffy said in a statement.

"Our country owes him and his family a great debt of gratitude for his ultimate sacrifice in service to our country. Today's plea is an important step in seeking justice on behalf of agent Terry."

The botched "Fast and Furious" operation, a US effort to track guns into Mexico that left drug cartels with high-powered weapons, was launched in 2009, but was suspended after the border agent's killing.

The aim was to build cases against gang members by knowingly allowing them to purchase assault weapons in the United States, then tracing those weapons to crime scenes in Mexico.

But most of the roughly 2,000 weapons were never found, while two of them that were showed up at Terry's murder site on December 14, 2010.

Under a plea deal, Osorio-Arellanes admitted that he and others were in the United States to rob drug traffickers of their contraband, and ended up in a firefight with border patrol agents.

The Mexican, who was arrested on the night of the shooting, faces up to life behind bars.

Federal Bureau of Investigation Special Agent in Charge James Turgal hailed the plea agreement as an "important step forward" and said the FBI was pursuing the remaining fugitives.

Turgal, in charge of the FBI's Phoenix office, praised Terry as a "hero" who served selflessly, vowing to leave "no stone unturned in holding all of those accountable."

The US Department of Justice offered a $1 million reward in July for information leading to the arrest of four men: Jesus Rosario Favela-Astorga, Ivan Soto-Barraza, Heraclio Osorio-Arellanes and Lionel Portillo-Meza.

In September, Portillo-Meza was captured in Mexico. He is currently awaiting extradition to the United States.

The men face charges including first degree murder, second-degree murder, using a firearm during a crime of violence, assault on a federal officer and possession of a firearm by a prohibited person.

In addition to murdering Terry, they are also alleged to have assaulted his fellow border agents William Castano, Gabriel Fragoza and Timothy Keller, involved in the firefight.