A Mexican national entered a guilty plea Thursday in the 2011 killing in Mexico of a U.S. immigration agent and the attempted murder of another.
A federal judge also unsealed guilty pleas already filed by three other defendants related to the same attack.
The man considered the ringleader, Julian Zapata Espinoza, will avoid the death penalty as part of an agreement reached with prosecutors that was announced in open court Thursday.
Mexico would not have allowed his extradition if U.S. prosecutors pursued capital punishment.
Espinoza was named in a four-count indictment handed up in April 2011. Prosecutors characterized him as a commander in the drug syndicate Los Zetas Cartel, and accused him of leading an ambush that ended with the death of Immigration and Customs Enforcement Special Agent Jaime Zapata.
The agent's parents were in the courtroom for Thursday's hearing to watch Espinoza admit to the crime.
A woman identified to CNN as Zapata's mother was weeping as she left the courtroom. She declined to comment when a reporter asked for her thoughts.
The three others linked to the attack also pleaded guilty to related charges, according to the documents unsealed after Espinoza's hearing had concluded. Two of the men admitted direct involvement and a third pleaded guilty to charges of conspiracy and accessory to murder and attempted murder of the two agents.
Zapata and his colleague, Special Agent Victor Avila, were ambushed in February 2011 as they drove to Mexico City in an armored SUV with diplomatic license plates. Both were assigned to the U.S. Embassy there.
Avila survived gunshot wounds and would have been likely to testify had the case gone to trial.
Investigators at the scene found more than 80 shell casings from AK-47s around the agents' vehicle, according to information received at the time by U.S. Rep. Michael McCaul of Texas.
Mexican law prohibits foreign agents or diplomats from carrying weapons or conducting law enforcement while on Mexican soil.
McCaul had quoted investigators as saying there were 10 to 15 attackers and at least two vehicles that forced the agents' SUV off the road as the ambush began.
U.S. officials later revealed one of the weapons used in the attack had originated in the United States, and was associated with a drug cartel that was smuggling weapons into Mexico.
"My client accepts responsibility," said defense attorney Ron Earnest. He told CNN outside the courtroom afterward that "there are witnesses, and they would be very convincing" if the case had gone to a jury trial.
Earnest said he advised Espinoza to admit to the crime after a "long period of reviewing the evidence" against him.
Espinoza, through a translator, told the judge he understood the charges, his potential punishment, and the meaning of admitting to the crime.
On the counts of murder and attempted murder of an employee of the U.S. government, he faces a mandatory life sentence and fines of up to $500,000.
Sentencing for Espinoza is set for September 20. U.S. District Judge Royce Lamberth accepted an additional condition from prosecutors that if Espinoza is ever released from prison it would lead to his deportation.
As part of the plea agreement, the judge at sentencing is to dismiss Counts 3 and 4 of the indictment, relating to the use of firearms in a crime of violence causing death, and the attempted murder of an internationally protected person.
No sentencing dates were set for the other defendants.