By Cheri Mossburg and Madison Park, CNN

(CNN) -- An 18-year old woman was stabbed to death at a public transit station Sunday evening in Oakland, California, marking the third death in less than a week from unrelated attacks on the Bay Area transit system known as BART.

On Sunday night, Nia Wilson was walking on the subway platform with her sisters when a man stabbed her in an unprovoked attack, according to the Bay Area Rapid Transit officials. Nia's sister Lahtifa, was also stabbed and seriously injured.

"In my close to 30 years of police experience, it was probably one of the most vicious attacks that I've seen," BART Police Chief Carlos Rojas said, citing surveillance video in a Monday press conference.

Police identified the suspect as John Cowell, 27, who was taken into custody Monday, according to BART police.

Rojas described Cowell as a violent felon, saying he was currently on parole after getting out of the California prison system four months ago. Surveillance video showed not just the stabbing on the platform, but the attacker removing his clothing in the parking structure as he fled, police said.

BART police officers were patrolling the MacArthur station on Sunday night when the two sisters were stabbed around 9:36 p.m. They rendered aid to the two victims, according to a BART news release.

As the search intensified for Cowell in the hours after the attack, BART police received a call Monday evening that a passenger had spotted the suspect on one of the trains, Rojas said. Officers were able to take Cowell into custody.

He was not armed when he was detained, Rojas said.

"We're very glad that less than 24 hours, we were able to get this suspect into custody," Rojas said.

It was unclear whether Cowell has a lawyer. Rojas said he had not yet been charged.

On Monday evening, marchers held a vigil for Wilson that brought nearly 1,000 people to downtown Oakland, according to an estimate by the Oakland police.

"The senseless and violent stabbing of two young women on a BART train platform last night has shaken our community," said Oakland Mayor Libby Schaaf in a statement.

"Although investigators currently have no evidence to conclude that this tragedy was racially motivated or that the suspect was affiliated with any hate groups, the fact that his victims were both young African American women stirs deep pain and palpable fear in all of us who acknowledge the reality that our country still suffers from a tragic and deeply racist history," she said.

Cowell is white, but police say no motive has been established. Cowell hasn't been connected to any radical right wing or white supremacist group, but police are investigating all angles because he "has a violent past," Rojas said.

Cowell was paroled in May after being sentenced to two years in prison for second-degree robbery, according to California Department of Corrections and Rehabilitation spokeswoman Terry Thornton.

Discussing the motive that it is so far unknown, Wilson's godfather Daryle Allums said, "We are asking the African-American community to stand down right now."

Attacks on the public transit system

Wilson's stabbing was the third death after other attacks in recent days on the BART system, which serves as the backbone of public transportation in the Bay Area.

Rojas discussed the two other incidents. In each case, the victim died after an assault at a BART station.

On early Saturday morning, a 47-year-old man was found injured on the platform at the Bay Fair Station, which is located in San Leandro, south of Oakland.

He was declared brain dead at the hospital. The victim, identified as Don Stevens, a transient, was punched at the side of his head by an unidentified suspect, according to BART surveillance videos. After being struck, Stevens fell to the cement ground on the platform. Authorities are searching for a suspect.

On Wednesday, a 51-year-old man was assaulted at the Pleasant Hill Station in Walnut Creek. The victim, identified as Gerald Bisbee suffered a minor bloody lip and small cut to the back of his knee, and went to the hospital the next day because he said he wasn't feeling well. The man went to the hospital on Thursday, and was later released. On Friday, he was found dead in his bed.

An autopsy found the cause of death came from an infection stemming from a small cut on Bisbee's knee, according to BART. The suspect in this case is in custody, according to BART.