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Police in California are investigating an incident involving a San Francisco Bay Area public transit rider who was caught on camera allegedly slapping another customer and using racial slurs.

Bay Area Rapid Transit (BART) Police received three calls around 10:10 p.m. PT on Monday from riders reporting a man threatening another customer.

A still taken from a video showing a man harassing an apparently Asian passenger on public transportation in the San Francisco Bay Area. Courtesy of Wiseley Wu

Officers boarded the train, where reports indicated the suspect had gotten off, BART said in a statement. But customers were unable or unwilling to identify the suspect, and no one wanted to press charges, police said.

Video of the incident uploaded online, which has been viewed more than 900,000 times across multiple platforms, begins with what appears to be a white man clad in an untucked shirt raising his fist at a seated passenger apparently of Asian descent and verbally threatening him. Soon after, a customer next to the Asian man gets up and moves away.

“I’ll punch you down in your seat,” the first man says, as the Asian passenger remains seated.

The man then slaps the Asian rider, the video shows. A few seconds later, he leans into the Asian man and uses a racial slur. He then hits the rider a second time.

At that point, the Asian passenger stands up, responding to the man and throwing down his headphones and bag.

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Other passengers can be heard saying, “Don’t do it,” and, “He’s not worth it,” as a woman steps in between the two men.

Efforts to reach the Asian man in the video were not immediately successful. In an interview with the San Francisco Chronicle, a person purporting to be the man said his name was Charles Wu and he engaged the first man after he used a racial slur.

"No one was saying anything and it was a BART full of people," Wu reportedly told the newspaper. "I had enough, so I told him, 'Sir, you need to stop talking."

Wiseley Wu, a 28-year-old who said he shot the video, told NBC News by phone Wednesday that he heard from the victim that the white man had allegedly been using a racial slur while on the train.

“He told him to stop, and this is how it started,” Wu, who said he did not know the victim, said.

The white man, who Wu said was in his 60s, then directed racial taunts at the Asian man, according to Wu.

“I didn’t start recording until he walked closer and closer,” Wu said.

Wu said he decided to video the encounter in case it was needed for evidence. He said he’s turned over the recording to police.

Wu, who is of Chinese descent, added that he wished he had done more.

“One thing I learned, if you see something, definitely don’t assume anything,” he said. “Call the police right away.”

BART Police said that while the video “captures an unfortunate, ugly scene,” they praised the response of riders, saying they voiced support for the victim without physically intervening and suggested contacting police.

Police said they were still pursuing their investigation despite seeing social media posts from the victim saying he didn’t want to press charges.

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