It started off like any other summer night: a group of friends lounging around in their regular hangout spot - a shed in the backyard -until things turned deadly.

Details of Thursday night's slaying of McAllen teenager Robert Reyna emerged Saturday morning following the arraignment of Anthony Mathew Reyna, his accused killer. A judge formally charged Anthony Reyna, 17, with murder - for which he could serve life in prison - and set his bond at $1 million. Anthony Reyna has no familial relation to the victim.

According to court records, a group of friends, which included juveniles, was hanging out at 9:54 p.m. Thursday at a home on the 2400 block of West Gumwood Avenue. Outside, Anthony Reyna, known as "TO" in the group, began roughhousing with 18-year-old Robert Reyna, known as "Kool Aid."

They continued to the point that Anthony Reyna became aggressive toward Robert Reyna, but the group calmed the two down and they all walked into the backyard shed, where the two wound up sitting next to each other, according to the probable cause affidavit in the case.

The two continued to roughhouse and then forcefully punched each other, to the point that Anthony Reyna became "extremely upset at Robert Reyna," the document says. During the altercation, the shed's lights were off, and it was illuminated only by a strobe light.

The two then stood up - apparently ready to fight each other - and one witness reported noticing that Anthony Reyna was holding a hammer in his right hand. Anthony Reyna swung the hammer, striking Robert Reyna in the left side of the head, according to witness accounts.

Robert Reyna fell back a few steps, and then Anthony Reyna punched him several times in the head, the affidavit says. Robert Reyna fell face-first to the floor, but Anthony Reyna allegedly continued to punch him in the back of the head, as Robert Reyna lay defenseless.

The group then restrained Anthony Reyna to stop the attack, police say, but Anthony Reyna ran from the shed toward an alley. One of the group members chased after him and dragged him back to the shed, where the group had turned on the lights and realized Robert Reyna was hurt.

After a struggle, Anthony Reyna fled the scene, according to the affidavit. The group called emergency responders after noticing Robert Reyna was bleeding.

The injured teen was transported to McAllen Medical Center, where he was pronounced dead at 10:47 p.m. The autopsy revealed that he died as a result of his head injury.

All of the witnesses provided statements at the police station identifying Anthony Reyna as the one who assaulted Robert Reyna.

McAllen police Sgt. Joel Morales said Anthony Reyna turned himself in to police late Friday afternoon. Nobody else in the group has been arrested in connection with the killing, but it remains an ongoing investigation, Morales said.

A man who answered the phone Saturday at the home where the altercation took place identified himself only as a friend of Robert Reyna. Although he declined to talk about the killing itself and abruptly hung up the phone, he did say Robert Reyna was friendly, enjoyed writing, didn't drink or do drugs and liked telling jokes and making people laugh.

Further attempts to contact the man were unsuccessful.