It was the start of an assault, police said.
The second man, who was white, didn't want to answer the question. Then the first man, who was black, boxed him in the face. Two more African-American men joined in the beating, according to a police report about Monday's incident.
It was caught on surveillance cameras on the MetroLink train and a passenger recorded it with a cell phone and posted the video online. It has gone viral.
Police confirmed to CNN that the online images came from the attack.
The victim, 43, was commuting home when a young man in a red T-shirt and cap walked up to him. The victim asked not to be named in media reports.
The man asked to use the victim's cell phone. He declined, and the young man sat down beside him.
"Then he asked me my opinion on the Michael Brown thing," the victim told KMOV, "and I responded I was too tired to think about it right now."
The suspect, in his 20s, stood up.
"The next thing I know, he sucker punches me right in the middle of my face," the victim said. The video showed the suspect unleashing a barrage of punches at the head of the victim, who covered himself with his hand and forearms.
The two other men, also in their 20s, joined in, police said. As the train pulled into a station, a security guard saw part of the beating and alerted police.
The man in the red T-shirt could be seen on video kicking at the victim's face before the train's doors opened and the assailants ran out.
Faces on camera
The train's surveillance cameras captured clear images of their faces, which MetroLink passed on to journalists. Police are looking for the three men.
They face possible charges of third-degree assault, police said. It is a misdemeanor under Missouri law
The victim was left with bruises on his face and forehead, police said. The first punch dug the frame of his eyeglasses into the skin of his nose, the victim said. He declined to receive medical treatment.
But then there was the emotional pain. On the video, people could be heard laughing while the man was beaten.
"I think it was disgusting that people were sort of laughing and smiling about it," the victim said. "And no one offered to help. No one seemed to call 911."
Condemnation of the beating has spread across social media, including from people who protested the shooting of Brown last year by a white police officer in Ferguson, Missouri. The officer was not indicted in Brown's death. St. Louis alderman Antonio French, a vocal community leader in the aftermath of Ferguson, tweeted his disappointment in the beating.
"Mike Brown question sparks MetroLink beating caught on video," he wrote. "This is disgusting. We have a major problem, STL."