Letifah Wilson, 26, the wounded sister of an 18-year-old woman fatally stabbed in the neck. Picture: KGO-TV via AP

Oakland, California -  The hunt for a man police say killed an 18-year-old woman in an unprovoked stabbing in the San Francisco Bay Area has ended where it began: In a train station.

Authorities arrested 27-year-old John Cowell on an Antioch-bound train on Monday night at a Bay Area Rapid Transit station about a dozen miles from Oakland.

Police say Cowell killed Nia Wilson with a knife and wounded her sister at an Oakland station on Sunday night.

BART Police Chief Carlos Rojas says the motive for the attack is still being investigated. 

An anonymous tip from a rider on a Bay Area Rapid Transit train led to the arrest of Cowell, 27, Rojas said at a news conference.

Cowell - a paroled felon - was arrested at another BART station in the San Francisco Bay Area. He had no weapon and was arrested peacefully, Rojas said.

A surveillance camera image shows John Lee Cowell. Picture: Bay Area Rapid Transit via AP

"This is the first step to achieving justice for Nia and her family," Rojas said.

A manhunt had been underway for Cowell since the Sunday night attack that killed Nia Wilson and wounded her sister on the platform at the MacArthur BART station.

Asked how a wanted man could have gotten back into the BART system, Rojas said he didn't immediately know but said the public transportation system is "porous" and large, serving four counties.

Surveillance video on the train and at the station's platform showed Cowell had been riding the same car as the sisters on Sunday, but they did not interact, Rojas said.

As the group got on the platform, Cowell attacked, the police chief said. "It looks like it was an unprovoked, unwarranted, vicious attack," Rojas said.

Rojas didn't provide any motive for what he called a "prison-style attack" in which Nia was stabbed twice in the neck.

Rojas said Cowell hasn't been connected with any white supremacist groups but added that "we are going to explore all options and all possibilities."

Nia and her sister, Letifah Wilson, 26, are black and Cowell is white.

Cowell was released from state prison on May 6 after completing a sentence for second-degree robbery, said Vicky Waters, spokeswoman for the California Department of Corrections and Rehabilitation.

Cowell pulled a knife and a replica gun on a supermarket clerk who confronted him over stealing, court records show.

Cowell also had prior arrests that include assault with a deadly weapon, the East Bay Times reported.

Letifah Wilson said that she, Nia and a third sister had been returning from a family outing Sunday when they were "blindsided by a maniac".

"I looked back and he was wiping off his knife and stood at the stairs and just looked. From then on, I was caring for my sister," Wilson told ABC7 News on Monday, speaking outside a family member's home with a bandage on her neck.

Surveillance footage showed a man police say is Cowell fleeing the station through a parking lot and stripping off his clothes there. Detectives recovered a knife they believed was used in the attack at a nearby construction site, Rojas said.

A memorial for 18-year-old Nia Wilson takes shape outside Bay Area Rapid Transit's MacArthur Station after she was fatally stabbed on a platform at the station, in Oakland. Picture: Lorin Eleni Gill/AP

Wilson said a woman with a stroller gave her a baby blanket to apply pressure to her sister's neck, but Nia Wilson died calling out her sister's name for help.

Wilson said she told her sister she loved her.

"We're gonna get through this, I got you, you're my baby sister," she said.

The women's father, Ansar Muhammad, said one of his daughters called him, crying hysterically, and told him to get to the MacArthur station.

"It's nothing imaginable, seeing your child on the BART platform with a yellow tarp over her body," Muhammad said Monday evening as he and other family members joined dozens of people for a vigil at the station. "That is an image I'll never forget for the rest of my life. So I want justice. All I want is justice."

Nia was "the baby" among six children, said her eldest sister, Unieve Wilson, 37.

She was "spunky, feisty, loved to dance," she said. "She wouldn't harm a fly."

Ebony Monroe, a cousin of the victims, said her cousin attended Oakland High School and loved her family, dancing and make-up.

"She was a beautiful person inside and out, and she didn't deserve it," Monroe said.

A man who witnessed the attack told them the suspect came out of nowhere, slit Nia's throat and stabbed her sister as she tried to stop him, she said.

"She died for no reason," Monroe said. "We want to see justice."

The killing was the third possible homicide on the BART line in just five days, accoring to the San Francisco Chronicle.

A homeless man at the Bay Fair Station in San Leandro died Sunday at a hospital, a day after he was punched by an attacker and fell down and hit his head on the pavement. He was identified as Don Stevens, 47.

Another man, Gerald Bisbee, 51, died Friday at his Pittsburg, California, home.

Bisbee was attacked by a man who was quickly arrested and suffered a bloody lip and a small cut to the back of his knee.

An autopsy Sunday determined that Bisbee died because the cut became infected, Rojas told the Chronicle.

The three deaths appear to be unrelated but they represent a worrying "anomaly," the police chief said.

"It's rare that we have a homicide, so of course this is ramped up to the highest priority when you have three deaths," Rojas said.