Imagine settling in for your morning commute on BART when a man yells this at the top of his lungs: “I’m going to kill you.”

Mike Hohndorf and dozens of other passengers got a body-shaking jolt, not unlike a train lurching out of a station, during their commute early Friday morning from a belligerent rider who terrorized a train car for seven stops — even though Hohndorf said the man first made threats at the North Concord/Martinez BART Station and that he told a BART police officer about the man at the station.

“I’m going to stab all of you,” the man yelled as the passengers clustered waiting for the station gates to open. The officer allowed the man to board the train, where he continued to threaten passengers. Hohndorf and another passenger contacted BART officials to report the problem. BART told them they would mobilize at the 12th Street Station in Oakland — several stops later. By then, the man had gotten off.

Hohndorf, a BART rider for three decades, has seen a lot interesting episodes and characters on trains over the years — from bloody fist fights to naked people. But this was the first time he felt scared.

A recent spate of robberies and other incidents has some BART passengers riding on the edge of their seats. In this latest case, the man who threatened people got off the train instead of killing anyone.

What if he had really pulled a knife?

Hohndorf, 60, believes BART police failed to take the disturbance seriously.

BART officials told me there was little they could do. They said that train operators notified BART officers of the complaints by passengers — but that no one described an imminent threat. The officials said they could act only on information they receive.

If that’s the case, then BART police and train operators need to ask more specific and probing questions when riders call authorities complaining about passenger behavior. Because a rider exhibiting volatile behavior should never be allowed to stay on a train.

Here’s one question every BART employee should ask a rider who seeks help on BART: Do you feel safe?

Hohndorf arrives at the North Concord/Martinez BART Station at around 3:30 a.m. every weekday. When he parked there Friday morning, he noticed a BART police cruiser tailing a man flailing his arms.

Hohndorf said the officer talked to the guy for a few minutes before driving away. When Hohndorf joined about three dozen others waiting for the station gates to open, the guy returned and screamed at the sleepy-eyed commuters.

“I’m going to kill all of you,” he said.

Hohndorf missed his train because he has a torn meniscus — the surgery’s Wednesday — and walks slowly with a cane. As he waited for the next train, he saw another BART police officer walking behind the man on the platform.

“I go, ‘You need to remove this guy. He’s a danger to himself and he’s a danger to others,’” Hohndorf recalled.

Hohndorf said the officer told him he’d get to the bottom of it. The train arrived.

“I look to the left, and I’m watching the guy get on the train with the cop standing there,” he said.

He found humor in the harrowing tale when we spoke Sunday morning before he and his wife went to brunch. Still, he wondered how a person shouting profanities and threatening to stab people could’ve been allowed to board the train.

The screaming continued on the train, and people left the car. Hohndorf stayed. The man walked toward Hohndorf, who sat in a seat for people with disabilities. The man carried a black plastic bag. He put his hand in the bag and told Hohndorf he was going to stab him.

Hohndorf called the train operator using the car’s intercom.

“He says to me, ‘I know. Other people have already called it in. The police will be boarding the car at 12th Street,’” Hohndorf recalled.

BART officials told me another passenger called police at the Orinda Station and said a man was “acting erratically on the train.” That’s when dispatchers began coordinating to intercept the train at 12th Street Station, where the closest BART police officer was stationed, according to Lance Haight, BART’s deputy police chief.

But the man got off at the MacArthur Station.

“We understand that when you’re in a public space and you’re riding BART, sometimes you’re on a train with someone that makes you feel uncomfortable,” BART spokeswoman Alicia Trost said. “That doesn’t mean necessarily that we can prevent that person from boarding.”

Haight said the first officer who spoke with the man at the North Concord/Martinez Station determined he wasn’t a threat.

“We can only take enforcement action when the person has either broken the law or if they’re a danger to themself or others, and that did not appear to be the case, based on what our officer saw in the parking lot,” Haight said.

But what about the second officer that Hohndorf spoke to on the train platform before boarding? According to Trost, Hohndorf only told the officer that the man yelled — but didn’t mention threats.

When he arrived at his office, Hohndorf called BART customer service to complain, and he was told he’d get a call back. But the call back didn’t happen until Tuesday morning, after The Chronicle had begun making inquiries.

“There’s a lot going on in our system, and we all have to look out for each other,” Trost said. “Safety is clearly the top priority.”

It has to be, because BART riders like Hohndorf shouldn’t worry about being terrorized during their commute.

San Francisco Chronicle columnist Otis R. Taylor Jr. appears Mondays, Wednesdays and Fridays. Email: otaylor@sfchronicle.com Twitter: @otisrtaylorjr