A Valencia County grand jury indicted the teen on charges of first-degree murder and tampering with evidence.
Thirteenth Judicial District Attorney Lemuel Martinez said last week that he intends to try the 15-year-old as an adult. But he said a judge would have the option of sentencing the teen as a youthful offender. That means the judge wouldn't have to impose a maximum sentence if the boy is convicted.
An arrest warrant affidavit said the 15-year-old hit Alex Madrid, of Albuquerque, on the left side of his head, then placed him under a mattress in a rural field at Meadow Lake, an area about 40 miles south of Albuquerque.
The teen told investigators he hit Madrid in self-defense because Madrid tried to stab him with a screwdriver following an argument over the teen's infant daughter, the affidavit said.
The 15-year-old's baby girl is 4 months old, deputies said.
Sheriff Louis Burkhard said Madrid appears to have died from severe trauma.
The boy's body was found last week in the field littered with trash and old tires in a rural, unincorporated area outside Los Lunas.
Under New Mexico law, the state can charge minors as adults only if they are at least 14. The AP typically doesn't identify juveniles charged in crimes.
At a recent detention hearing, Cindy Mercer, the suspect's attorney, denied the allegation that the teen killed the 12-year-old and suggested her client can't comprehend what's happening.
"Due to his low mental functioning, I don't believe he understands the charges or his rights," she said.