DUBLIN, Calif. (KRON) — What would motivate a young Alameda County Sheriff’s deputy to break into a married couple’s house and murder them execution-style?

Sheriff’s deputy Devin Williams Jr., 24, had just finished working a night shift at the county jail in Dublin at 11 p.m. on September 6 when he drove to a nearby house at 3112 Colebrook Lane, investigators said.

At 12:45 a.m., Williams broke into the house brandishing his service weapon and found his targets, according to court records. The deputy shot 42-year-old Maria Tran and her husband, Benison Tran, in the head and neck.

Maria Tran’s horrified brother witnessed the double homicide and called 911. When Dublin Police Department officers and deputies arrived at the crime scene, one of the victim’s family members told them that the killer was “Devin. He is a cop.”

Sheriff’s Lt. Ray Kelly told KRON4 that investigators are retracing Williams’ life to unravel what went wrong in Williams’ life and mind that drove him to kill two people.

“There is no excuse for this. It’s not rational. He is a coward. He’s cold-hearted. This thing has rocked our Dublin community and the Sheriff’s Office,” Kelly said.

The deputy’s co-workers in the Alameda County Sheriff’s Office saw no signs that Williams had been struggling mentally, nor that he was prone to snap. “When we hired him a year ago, there were no indications that he would be capable of committing a double murder. So we are looking back over the last several months where his troubles began,” Kelly told KRON4 on Tuesday.

Investigators found a romantic connection between the deputy and Maria Tran, who worked as a nurse at John George Psychiatric Hospital in San Leandro.

“He was having a romantic relationship with the female victim. We’re looking into that relationship to see what was going on there,” Kelly said.

Dublin police detectives interviewed Williams’ father, who revealed that his son was in a “dating relationship” with Maria Tran and she had visited their Stockton home, court records state.

Maria Tran lived in a 5-bedroom house in an upscale Dublin neighborhood with their son. Williams’ mother told one news outlet that Maria Tran claimed she was a 35 years old single mom.

While Maria Tran and her husband died at the shooting scene, the deputy went on the run. A 12-hour manhunt ensued and Williams was described by law enforcement as “armed and dangerous.”

Williams fled 160 miles south to Coalinga, Calif., before Dublin Police Chief Garrett Holmes convinced him over the phone to surrender without hurting himself or anyone else. Williams was arrested on charges of first-degree murder and burglary. The deputy was booked into the same jail where he had worked just the day before. He invoked his Miranda Rights, the right to remain silent.

“We are still in shock. Everybody is having a hard time with this as the reality of it sinks in. The family is struggling, they are having a very difficult time. We will continue to support them and this community,” Kelly said.

The Alameda County Sheriff’s Office is looking closely at Williams’ hiring and 1-year on duty to see if any possible red flags were missed. The deputy grew up in an affluent home, graduated from college with honors, and performed well on the job.

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“We vet our candidates very, very stringently. We are trying to figure out, where did his life go off kilter?” Kelly said.

Homicide detectives reconstructed a timeline of the deputy’s activities and whereabouts the day before and the day of the killings. Newly uncovered evidence helped fill a hole in the timeline between when Williams completed his shift at Santa Rita Jail at 11 p.m., and when he broke into the Trans’ home.

Investigators believe that the deputy was likely prowling outside the Trans’ house and monitoring who was inside until he decided to break in. Police said there were six people inside the home, including the victims, their son, and Maria Tran’s brother.

The victim’s brother was on the phone with 911 emergency dispatchers when the deputy allegedly pulled the trigger. Six gunshots were audible in a recording of the 911 call.

Detectives recovered six firearm shell casings with “Speer 9 mm” inscribed on them. The Alameda County Sheriff’s Office uses Speer 9 mm ammunition, according to court documents. Investigators suspect that the deputy threw his service weapon out of his car window before he surrendered.

Williams is scheduled to appear in the East County Hall of Justice Wednesday to enter a plea. He is being held in Santa Rita Jail without bail.