"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 prosecuting 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 U.S.
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 bought 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's separate from the prosecution into Terry's death.
Avila, who isn't charged in Terry's death, faces up to 10 years in prison when he's sentenced on December 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. The FBI said it continues to aggressively pursue the three fugitives.
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