BART said officers identified Cowell from surveillance images captured by onboard and station platform cameras. They described Cowell as white, 5 feet 8 inches tall, weighing 190 pounds, with short, dark hair and a closely cropped beard and mustache.

Rojas said Cowell, who was described as a transient, is "a violent felon who is currently on parole." He said officers had found a knife they believe to be the weapon used in the attack from a construction site adjacent to MacArthur Station.

BART police said Cowell should be considered dangerous and anyone who sees him should immediately call 911.

Rojas said the motive for the attack was unknown.

Daryle Allums, Nia Wilson's godfather and a member of Oakland's "Stop Killing Our Kids" anti-violence group, appeared with Rojas at the media briefing. He said Nia Wilson and her sister "didn't ask to get cut or stabbed. Those are baby girls. Those are our children."

In the midst of social media threads that have pointed to a possible racial motive for a white suspect's attack on black women, Allums said, "We need the community, especially the African-American community, to stand down. ... I'm in fear right now. We don't know if this was racist, we don't know if it was random, we don't know what it was."

He said Stop Killing Our Kids would hold a vigil at MacArthur Station from 4 p.m. to 7 p.m. Monday.

In answer to a reporter's question, Rojas said police don't have any information connecting Cowell to hate groups.

"But that's something we haven't taken off the table, and we'll look into that," he said. "I know there's been a lot of conversation on social media regarding that, but I don't have one piece of evidence that this is race-related."

BART Discloses Fatalities in Two Other Incidents

BART police also reported Monday that they were working to identify a suspect seen in a fatal assault early Saturday at Bay Fair Station in San Leandro. They said that at 1:20 a.m. Saturday, officers responded to a report of a man injured on the station platform.

The transit agency said a review of surveillance images showed that a man struck Don Stevens, 47, on the side of the head with a closed fist after an argument. Stevens struck his head on the concrete floor of the platform.

"The best way I could describe it is cold-cocking him in the side of the head," Rojas said. "It's completely unexpected."

Police officers performed CPR until paramedics arrived, BART said, but Stevens was declared brain-dead at a local hospital. He was removed from life support and died Sunday afternoon.

BART police describe the suspect as a black male, 5 feet 8 inches tall with a medium build, shoulder-length dreadlocks in a ponytail, and a scruffy beard. The suspect was wearing a white T-shirt and a green jacket at the time of the incident.

In a third incident, BART reported that Gerald Busbee, 51, of Pittsburg, died last week two days after suffering what appeared to be minor injuries in an altercation at the Pleasant Hill Station with a fellow passenger identified as Abdul Bey, 20.

Rojas said that Busbee was injured in a fight last Wednesday apparently provoked by what he described as "mad-dogging" -- a staredown -- between the two passengers. Busbee suffered a cut lip and a cut on his knee that Rojas described as about one-eighth of an inch long.

Bey was arrested after a lengthy foot pursuit, Rojas said. He was charged at the time with one count of battery on a transit passenger and five counts of resisting arrest.

Busbee later went to a local hospital, complaining he wasn't feeling well, Rojas said. He was sent home after an examination, but was found dead in bed at his home on Friday.

The BART chief said an autopsy had determined that the cause of death appeared to be an infection arising from the cut on Busbee's knee. He said the investigation is ongoing and BART police are conferring with Contra Costa authorities about charges in the case.

The three fatal incidents in less than a week appear unprecedented in BART history. The last incident in which a BART passenger died as a result of an attack by a fellow rider occurred in January 2016.

That's when Carlos Misael Funez-Romero, 19, of Antioch was shot and killed aboard a San Francisco-bound train pulling into the West Oakland station.

Prior to that killing, the most recent homicide aboard BART was believed to have occurred in the late 1990s.

The Funez-Romero killing has not been solved. However, the incident brought to light the fact that virtually all of the "cameras" aboard BART's train fleet were actually decoys.