SANTA ANA – Mechanic Agustin Villegas, 32, was on the 91 freeway in Anaheim, heading home from work with his 13-year-old son sitting in the back seat, when a stranger in a black car suddenly shot him in the head.

With no suspect description and just a single shell casing found at the scene, police had little evidence – until, a prosecutor said Wednesday, an informant gave them a lead.

Trial began on Wednesday for David Arzate Cabrera, a 47-year-old who authorities say was a cocaine dealer and has said he shot Villegas because he thought the victim had followed him.

In his opening statement, Senior Deputy District Attorney Steve McGreevy said Villegas was repairing a car in Anaheim with his cousin at an auto-repair shop on July 14, 2014, when he headed home for dinner at about 8 p.m.

Villegas was in the passenger seat, with his cousin driving. Each had teenage sons in the back seats, out of school for summer, with them.

Near the 57 freeway interchange, a bullet shattered a side window and grazed the cousin’s head before striking Villegas in the head.

He quickly died.

“His son was sitting in the back seat, feet away, helpless as his dad was killed right before him,” the prosecutor said.

Anaheim police had only a vague description of a black car.

But later an informant said a drug dealer named “Dave” had told him that he had shot someone on the 91, McGreevy said. Police set up a controlled drug deal between the informant and Cabrera, who was already the target of a drug-trafficking investigation, authorities said.

After arresting Cabrera, they searched his home in an Anaheim trailer park and found a black Nissan in the driveway with a secret compartment in the center console, McGreevy said. The compartment held $15,000 in cash, 15 grams of cocaine, and a Glock pistol with the same type of ammunition used in the shooting, he said.

When interviewed by a detective, Cabrera admitted to the shooting, saying that he believed the man had been following him around different locations that day and was trying to rob him, McGreevy said.

Cabrera’s defense attorney, Michael Garey, declined to give an opening statement on Wednesday.

If convicted, Cabrera faces a sentence of life in prison without the possibility of parole.