OAKLAND — A BART police officer killed during an apartment search Tuesday died from a single gunshot wound after a lone round passed through a small gap in the bulletproof vest he was wearing — an extraordinarily rare outcome that compounded the tragedy of his death by friendly fire.

An autopsy performed on Sgt. Tom Smith Jr., 42, showed the lone bullet missed the 23-year veteran’s vest and struck him in the chest, said Sgt. J.D. Nelson, an Alameda County Sheriff’s Office spokesman who referenced a pathologist’s initial findings in a statement Thursday.

“Wearing a vest doesn’t always afford someone 100 percent protection,” said Nelson, who did not disclose the exact location where the bullet struck Smith. Most bulletproof vests have small areas — at the base of the neck and under the arms — where they do not completely cover an officer’s torso.

Studies have shown that deaths like Smith’s are uncommon: A 2002 congressional report estimated that the risk of dying from gunfire is 14 times higher for an officer not wearing a ballistic vest. An FBI report found that of 23 police officers who were killed with a firearm while wearing a bulletproof vest in 2012, only three were struck somewhere other than the head.

Bay Area police officers say that even with the added protection of a bulletproof vest, risks await them when they go out on duty.

“Every moment of every day … the fear of getting shot is there,” said Richmond police Sgt. Nicole Abetkov. “It is something that is always in the back of your mind. You just never know.

“(Vests) are a necessity, obviously, but the thing about it is that they don’t stop every type of round. They have to fit properly. It’s an added security measure.”

Although the Sheriff’s Office confirmed Smith’s cause of death Thursday, investigators have refused to publicly address how the shooting unfolded — particularly whether the other officer mistook Smith for a threat or if his handgun went off accidentally.

Nelson said seven BART officers and a Dublin police officer converged on the robbery suspect’s apartment on Dougherty Road around 2 p.m. Tuesday. They knocked twice on the door, but no one answered.

Finding the door unlocked, five BART officers, including Smith, stepped in. All five officers were wearing bulletproof vests, investigators have said; all were in plain clothes, not uniforms.

Not knowing whether anyone was inside the apartment, officers followed protocol and entered with their guns drawn, Nelson said.

Investigators have not disclosed what happened next. After the shooting, Smith was taken to Eden Medical Center in Castro Valley, where he was pronounced dead.

Addressing a very somber meeting of the BART board of directors Thursday morning, BART police Chief Kenton Rainey urged the directors to wait for answers on the fatal shooting of Smith until the internal investigation is complete.

“Obviously, something went wrong and we will not hide from the truth,” Rainey said. “Whatever comes out, comes out. We will make whatever necessary changes.”

At the pressing of BART Director Robert Raburn, the chief confirmed that two uniformed BART officers on the scene had video cameras on their lapels. The video was turned over to the Sheriff’s Office. The chief said he didn’t know if the cameras contained data but that the video would be used in the internal investigation, as well as separate probes by the Sheriff’s Office and the Alameda County District Attorney’s Office.

“As in any case in which an officer-involved shooting results in the death of a person, the DA’s office conducts a parallel investigation,” Alameda County District Attorney’s Office spokeswoman Teresa Drenick said. “In this case we are conducting an investigation. The investigation is ongoing, and I cannot comment further.”

Rainey said he had spoken to Smith’s family and the officer who fired the fatal shot. He described the officer as “doing well but upset.”

Rainey has said he will not name the officer who shot Smith until next week, to give him and his family time to cope with the tragedy. That officer has been placed on administrative leave with pay for an undetermined amount of time, Deputy Chief Benson Fairow said Thursday.

According to the department’s policy manual, there is no set time frame that he is required to return to duty. BART also makes mental health counseling available to employees in cases such as this.

Sources have said that Officer Michael Maes is the colleague who shot Smith. Maes’ attorney, David Mastagni, said his client has complied with every aspect of the ongoing investigation into Tuesday’s shooting. Mastagni declined to comment on the specifics of the case.

“The officers involved were both good friends,” the Sacramento-based attorney said. “The police community is like a family … the person you work with is the closest part of your family. This is a tragedy from everyone’s point of view.

“(Maes’) deep concern is for Officer Smith’s family,” Mastagni said.

Smith was the first officer killed in the line of duty in the BART Police Department’s 42-year history. He leaves behind his wife, Kellie — also a BART officer he met on the job — and their 6-year-old daughter.

Staff writers Lisa Vorderbrueggen, Katie Nelson and Malaika Fraley contributed to this report.