Reta was sentenced to 40 years in prison by Judge Joe Lopez immediately after he entered the plea, which could have brought a sentence of up to 99 years.

Reta's attorney, Eduardo Pena, said the American teen, who was born in Houston but grew up in Laredo, decided to plead guilty — with the right to appeal — after a statement he signed was admitted into evidence over Pena's objections. In the statement, he admitted being the driver of the car used in the murder of Noe Flores.

"Under the circumstances, it was the best we could do," Pena said of the plea.

A witness testified Wednesday that Rosalio "Bart" Reta was among three men in an organized cell paid $15,000 to kill Flores in January 2006.

Prosecutors say Reta and two other accused hit men were actually supposed to kill the half brother of Flores but mistakenly killed Flores instead. Flores was shot at least seven times from the back while standing in front of a Laredo home.

Reta, a baby-faced teen with dimples, was only facing charges in Flores' death this week, but he is separately charged in another Laredo killing, also allegedly carried out on the orders of the cartel.

Assistant District Attorney Jesse Guillen said that trial would likely start soon and would give authorities another chance to add time to the teen's sentence. Laredo investigators believe Reta began killing for the cartel in Mexico and was moved to Laredo when the cartel began placing operations on the U.S. side.

Despite Reta's youth and his 5-foot-2 stature, "he's a cold-blooded killer," Guillen said. "There's no doubt about it."

During testimony Wednesday, Laredo police Detective Robert Garcia laid out a pattern of phone records that connected Reta to two other members of his cartel-controlled group and suppliers of the car and guns used in the Flores killing.

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.