OAKLAND — From the start, the death of a police officer’s wife inside an Oakland apartment had the makings of a homicide investigation, with Officer Brendan O’Brien the prime suspect.

O’Brien consented to a search of his home, car and cellphone. He gave a blood sample. Later at police headquarters, two detectives grilled him about previous arguments with his wife.

Investigators eventually ruled the June 2014 death of Irma Huerta Lopez a suicide, despite lingering suspicion from her family. O’Brien, who also committed suicide in September 2015, was again absolved on Monday after a lengthy review of the case by the office of Alameda County District Attorney Nancy O’Malley.

The DA’s report, while offering fresh details of the police investigation, confirmed Huerta Lopez’s death a suicide. Mayor Libby Schaaf had requested the new investigation after news reports said the gun she used was fired twice, deepening her family’s doubts over her death.

The new investigations came amid an East Bay-wide law enforcement sexual misconduct scandal exposed in a suicide note left by O’Brien. His suicide was also re-examined.

“After an exhaustive review of the investigation, it is our opinion that both deaths were investigated professionally and thoroughly, ultimately reaching the reasonable and appropriate determination to classify each death a suicide,” O’Malley wrote in a letter sent Friday to Schaaf, City Administrator Sabrina Landreth and assistant police Chief David Downing.

O’Brien’s suicide note — a two-page, typewritten letter signed by the 30-year-old officer according to the DA’s report — revealed the names of some officers who were involved with a woman who goes by the name of Celeste Guap.

Guap, an exploited teen who is the daughter of a police dispatcher, claimed to have had a relationship with O’Brien when she was underage. She has since said she had sex with about 30 officers from various departments, some encounters when she was underage. She also said some of the officers shared confidential information.

Guap’s allegations sparked a widespread investigation, resulting in several officers in Oakland, Richmond, Livermore, the Contra Costa Sheriff’s Office and an inspector with the Alameda County District Attorney’s Office being placed on leave or resigning.

In June, the fallout from the scandal caused Oakland police Chief Sean Whent to resign under pressure, and his two successors departed within one week, leaving the city without a chief of police.

The only possible reference to Guap in the DA’s report was the police interview of an 18-year-old “Jane Doe” five days after O’Brien’s suicide. Guap turned 18 about a month before O’Brien’s suicide on Sept. 25, 2015.

The bulk of deputy district attorney Timothy Wagstaffe’s report focused on Huerta Lopez’s death on June 16, 2014. O’Brien and a neighbor each called 911 at about 9:52 p.m. Paramedics pronounced Huerta Lopez dead at 10:13 p.m. O’Brien told police he left the house to buy cigarettes and returned to find his wife dead from a gunshot wound. A receipt from the gas station found in his car confirmed the purchase at 9:45 p.m. Surveillance footage obtained by police showed him in the store around the time of his wife’s suicide.

Over the next 12 hours, crime scene technicians took about 1,000 photographs as they searched the apartment, collected gunshot residue from O’Brien and Huerta Lopez and other evidence as officers interviewed neighbors in the apartment complex at on Greenridge Drive. Police said both O’Brien and Huerta Lopez had gunshot residue on their hands, but that her DNA was more prevalent on the weapon. Investigators said the residue on O’Brien could have come from earlier firing or handling a gun.

Meanwhile, O’Brien sat in an interview room at police headquarters from 12:21 a.m. until he was interviewed at 5:19 a.m.

During his interview with Sgts. Cesar Basa and Mike Gantt, “the investigators employed interrogation tactics, including the use of a ‘ruse’ in order to try and obtain information from Officer O’Brien,” Wagstaff wrote. “The investigators delved into any perceived inconsistencies in his statement until they were satisfied that all their questions had been answered truthfully and consistently.”

Nothing in O’Brien’s suicide note contradicted his statements to Basa and Gantt, according to the report. Gantt, a seasoned detective, was recently investigated for having his girlfriend transcribe his notes in a 2013 homicide case.

“This third-party review serves to reinforce the integrity of our police investigations,” Schaaf said in a statement. “The public deserves to have confidence that we take any allegations or wrongdoing seriously and that our police department continues to operate with the highest level of professional and ethical standards.”

Lt. Roland Holmgren, head of the homicide unit, said his unit takes all investigations seriously.

“I hope the results of the independent investigation will work to ease some of the doubts cast about our ability to conduct fair and unbiased investigations,” Holmgren said.

The DA’s office and the police department continue to investigate the allegations of sexual misconduct by officers.

Contact David DeBolt at 510-208-6453. Follow him at Twitter.com/daviddebolt.