LOS LUNAS, N.M. (AP) — A 15-year-old New Mexico boy accused of killing his 12-year-old friend struck him following an argument over the teen’s infant daughter then stuffed his body under a mattress in an abandoned field, court documents said.

The arrest warrant affidavit filed Wednesday said the suspect told investigators he hit Alex Madrid, of Albuquerque, in self-defense because Madrid tried to stab him with a screwdriver.

The teen told deputies he struck Madrid on the left side of his head and left him in a rural field outside Los Lunas, the records said.

The suspect’s daughter is around 4 months old, Valencia County Sheriff’s Capt. Gary Hall said.

The warrant, first obtained by KRQE-TV, didn’t elaborate on the argument over the baby. It also provided no additional details about how the suspect hit the victim, including whether a weapon or other object was used.

Sheriff Louis Burkhard said Madrid appears to have died from severe trauma.

The boy’s body was found Tuesday in the field littered with trash and old tires in a rural, unincorporated area known as Meadow Lake, about 40 miles south of Albuquerque. He was last seen Sunday with the 15-year-old and his family, Burkhard said.

The teen has been charged with first-degree murder and tampering with evidence.

His attorney, Cindy Mercer, didn’t immediately return a phone message from The Associated Press.

But at a detention hearing Thursday, Mercer 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.

According to the arrest warrant, the 15-year-old told investigators that he and Madrid went to vandalize an abandoned mobile home and were attacked by three males. The teen said he hit one of the attackers with a crowbar and fled the scene, the documents said.

But Burkhard said much of what the 15-year-old told deputies seemed like “wild tales,” and the teen later led authorities to the body.

After investigators confronted the teen about contradictions in his stories, he “became very emotional and stated that he was going to jail for a very long time,” the arrest warrant said.

The teen was then handcuffed and told investigators: “You want the truth? I only hit him once,” according to the affidavit.

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.