"There was a verbal exchange," Sheriff's Transit Det. Keith Schumaker said. "Then the suspect produces a weapon and lunged toward the victim with a stabbing motion."
"It was something more than a conversation. It wasn't anything that amounts to provocation. This was nothing like self-defense. There was a sudden attack after a few words," Schumaker said.
The video captured a limited view of the attacker, Schumaker said. The victim is in his 30s and was stabbed about 9:18 a.m. by another rider described as 18 to 24 years old. The victim was taken to a hospital in grave condition after the train stopped at the Vermont/Santa Monica Station.
Investigators are reviewing a second video and talking to witnesses.
Earlier Monday, a woman told The Times in a telephone interview that she witnessed the entire incident unfold Monday morning, calling it "quite traumatic."
The woman, who asked not to be identified out of fear of retaliation, said she boarded the train about 9:16 a.m. at the Vermont/Sunset station and was standing by the door when she saw the suspect, who appeared to be in his 20s, singing loudly in the car.
He "walks down the aisle to the middle section and was standing there singing really loud rap music," the woman said. "I particularly noticed because I was watching, thinking, that's aggressive and loud."
Then a man started walking toward the center where the man was singing.
"There was no provocation whatsoever and this kid just stabs the man in the neck," the woman said.
After that, the train stopped and the assailant ran out the door, she said. The victim was bleeding profusely, she said.
Marc Littman, an MTA spokesman, said that "the incidence of serious crimes aboard the Metro rail system is very low, about 0.30 per 100,000 boardings."