A killing on a BART train in West Oakland wasn’t the result of a sudden encounter between the gunman and his victim.

Investigators looking into the brazen Jan. 9 slaying have discovered not only that the suspect and victim both boarded at the Pittsburg/Bay Point Station 30 miles away, but that they rode the same Tri Delta Transit bus to the station, BART officials said Monday.

The bus carried passengers to Pittsburg from Antioch and other East Contra Costa County cities, and the two men had some sort of “interaction” that apparently continued until the 7:40 p.m. shooting, BART said.

Tri Delta Transit buses are outfitted with security cameras that captured images of the two men, but officials would not say what else the cameras captured that day.

What remains unclear is the relationship between the men, if any, and the motive for the killing.

Police continue to hunt for the suspect after releasing a series of surveillance photos of him — both in train stations and on the bus — and are speaking to the family of the victim, 19-year-old Carlos Misael Funez-Romero of Antioch.

“We know they were on the same bus. There was an interaction between the suspect and the victim that carried over into the BART system,” said Alicia Trost, a transit agency spokeswoman. “Investigators are pulling all video from the bus, and officers are talking to passengers, trying to find additional witnesses.”

When it released photos of the suspect on the bus last week, BART blurred out the backgrounds of the images. Trost said the agency did not at that time want to potentially tip off the suspect that investigators knew he had been on the bus.

Police said the gunman fired multiple shots at Funez-Romero from close range on a crowded, San Francisco-bound BART car as it pulled into the West Oakland Station.

The suspect, a black man who is tall and thin with close-cut hair, appeared to be wearing a dark green jacket with a hood, a backpack, jeans and beige work-style boots.

Investigators do not have video footage of the killing — and what may have immediately precipitated it — even though BART had what appeared to be cameras just feet from the killer and the victim on the train.

The vast majority of those devices are actually decoys, The Chronicle revealed last week. BART Police Chief Kenton Rainey defended the use of the dummy cameras, saying the pictures BART released of the suspect — taken by more modern station cameras — prove the security system functioned as intended.

Officials said the cameras inside trains, which were installed starting in 1998, were originally intended to deter vandals — and were extremely effective. They said every car in BART’s new fleet will be equipped with cutting-edge cameras producing footage that can be watched live from a central monitoring station. Those cars are expected to arrive between 2017 and 2021.

Evan Sernoffsky is a San Francisco Chronicle staff writer. E-mail: esernoffsky@sfchronicle.com Twitter: @evansernoffsky