Rosalio "Bart" Reta, 17, was immediately sentenced to 40 years in prison. He still faces separate charges in another killing in Laredo that was allegedly carried out on the orders of the Mexican Gulf Cartel.
The Webb County Courthouse was heavily guarded. Attorneys and witnesses have reported being threatened by the cartel's enforcers, and Reta has been moved into solitary confinement.
Witnesses testified Wednesday that Reta and two other men working for the cartel were paid $15,000 to kill a Laredo man in January 2006. Phone records linked the hit men, the ring leader and suppliers of the guns and car used in the shooting, Detective Robert Garcia testified.
Prosecutors say the shooting victim, Noe Flores, was the half brother of the intended target. Flores was shot at least seven times.
Reta decided to plead guilty, but retain his right to appeal, after his statement to police was allowed as evidence over his attorney's objections, said the attorney, Eduardo Pena. In the statement, Reta admitted being the driver of the car they used to go kill Flores.
"Under the circumstances, it was the best we could do," Pena said of the plea.
Assistant District Attorney Jesse Guillen said the trial in the second case against Reta would likely start soon and would give authorities another chance to add time to the teen's prison sentence.
Despite Reta's youth, "he's a cold-blooded killer," Guillen said. "There's no doubt about it."
The accused ring leader in the Flores killing, Jesus "Jesse" Gonzales, fled to Mexico after making bail. The third member of the group, Gabriel Cardona, pleaded guilty and is serving an 80-year sentence.
Authorities say Reta worked on both sides of the border for Miguel Trevino Morales, the leader of the cartel's operation in Nuevo Laredo, Mexico. He was allegedly part of a "cell" of young enforcers that followed rivals and tracked members of the cartel's own ranks.
Reta, a U.S. citizen, was extradited last year from Mexico to face charges in Flores' death. Authorities said he requested extradition after being arrested in connection with an explosion that killed four people in a crowded bar in Monterrey, Mexico.
Emanuel Arriaga, a spokesman for the Nuevo Leon state Attorney General's office, said Wednesday that prosecutors in Monterrey couldn't link Reta to any killings.