Oakland, California (CNN) Nia Wilson was standing on a Bay Area Rapid Transit station platform in Oakland, California, Sunday night when she was stabbed to death in an apparently unprovoked attack.

By Monday night, John Cowell, 27, had been arrested in connection to the stabbing, but days later, officials still haven't said what prompted the attack, which a police chief compared to a "prison yard assault."

The random murder of Wilson -- a black teenager described by her family as loving and selfless -- prompted nearly 1,000 people to march in Oakland in a vigil for Wilson early this week, according to an estimate by the Oakland police.

Some have claimed the attack was racially motivated, because Cowell is white, though the BART police and the Alameda County District Attorney's Office have said they don't have evidence to back up that claim. Cowell's family released a statement that said he had been struggling with mental illness.

But Wilson's family members and others in the community feel the murder should be considered a hate crime, regardless of what caused the fatal stabbing.

"It is a hate crime," Malika Harris, Wilson's 25-year-old sister told CNN on Thursday. "That's how I feel, okay?"

Nia Wilson was stabbed to death in an attack at a Bay Area Rapid Transit station on Sunday night.

She described her 18-year-old sister as "beautiful inside and out," and "always positive." She had been working at a clothing consignment store, but was thinking about joining the Army. Wilson, Harris said, was always willing to help people, no matter the circumstances.

Wilson's godfather, Daryle Allums, echoed that sentiment, and told CNN that Wilson had a "love for people."

"A lot of kids are selfish nowadays," he said. "She'd give her shirt off to a person who didn't have a shirt, you know?"

"My daughter was everything," Wilson's father, Ansar Muhammed, said, according to CNN affiliate KRON . "She was so beautiful, so inspirational, had dreams, you know?"

"All I want is justice for my daughters."

Malika Harris places a candle down for her sister Nia Wilson at a makeshift memorial outside the MacArthur Bay Area Rapid Transit station, Monday, July 23, 2018, in Oakland.

The attack

Wilson and her 26-year-old sister Lahtifa Wilson were transferring at the MacArthur BART station when Cowell allegedly walked up and stabbed both of them, according to police.

Lahtifa, who survived, described the attack to CNN affiliate KGO on Monday, and said it came out of nowhere.

The sisters were on their way home from a family function, she told KGO, when they were "blindsided by a maniac."

"For what, I don't know," she added.

"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."

Jasmine Malone, 23, holds a sign at a vigil held for 18-year-old Nia Wilson, who was stabbed to death the night before at the MacArthur BART station in Oakland, California, on Monday, July 23, 2018.

BART police were patrolling the station when the sisters were stabbed around 9:36 p.m., and rendered aid to them, according to a BART news release.

But Nia Wilson died at the scene, according to a probable cause document for Cowell's arrest. And her sister was taken to a hospital.

"It was, in my close to 30 years of police experience, probably one of the most vicious attacks that I've seen," said BART Police Chief Carlos Rojas in a news conference Monday morning.

Suspect 'has a violent past'

By Monday morning, BART police had identified Cowell as the suspect in the stabbing, and released images from past BART surveillance footage to help in the search.

According to the declaration of probable cause, Cowell was seen in surveillance footage stabbing the sisters before running away. As he escaped through a nearby parking structure, he discarded his sweatshirt and backpack, which was later found and contained items inside identifying him by name and his date of birth, it said.

On Monday evening, a BART rider recognized Cowell after seeing him on the local news, and called police. Soon after, he was in police custody.

Cowell has not been linked to any radical right wing or white supremacist groups, Rojas said, but he "has a violent past."

John Lee Cowell

Cowell was convicted of second-degree robbery and assault with a deadly weapon in 2016, according to the criminal complaint. He was paroled in May after being sentenced to two years in prison for second-degree robbery, according to California Department of Corrections and Rehabilitation.

Rojas described Cowell as a "transient," and said, "there's no doubt in my mind that this individual is the killer, and we're going to bring him to justice."

Cowell and the Wilson sisters had been on the same train car, but they never interacted, Rojas said, and there was nothing to suggest that there was any dispute between them.

"It looks like it's an unprovoked, unwarranted, vicious attack," Rojas said.

On Wednesday, Cowell was charged with the murder of Nia Wilson and the attempted murder of her sister. He's currently being held in the Santa Rita Jail without bail, according to online records.

CNN was unable to reach an attorney for Cowell on Friday, but attorney Jeff Chorney with the Alameda County Public Defender's Office told CNN earlier this week it was not commenting on the case at this time.

Was the attack racially motivated?

Cowell's family released a statement extending its sympathy to Wilson's, and said Cowell had long been suffering from mental illness.

"He has been in & out of jail & has not had the proper treatment," the statement said. He's been diagnosed with bipolar disorder and schizophrenia, the family said, and they had to get a restraining order at one point "for our own protection." Cowell's been living on the streets since.

"This is in no way an excuse for this senseless and vicious attack," the statement said.

But Wilson's father expressed doubt about the suggestion that Cowell's mental illness was behind the attack.

"Basically, I think it's an excuse," Ansar Muhammad told KRON. "I hate to say that but you know, why would you choose two young black girls? That's all I want to say."

Nia's sister Malika Harris agreed.

"He wasn't mentally ill. He was well-aware of everything that was going on," she told CNN. "If he was mentally ill, after the incident, why go change your clothes? Why would change your clothes, wipe off the knife?"

Members of the community had a range of opinions at a makeshift memorial for Wilson outside the MacArthur station on Thursday.

"I'm mad as hell," said Kayla Humphreys, a 20-year-old who went to the same high school as Wilson. "She's black. He's white. He knew exactly what he was doing."

Others, like Cedric Hunter, 35, aren't so sure. "It was a senseless act," Hunter said. "I don't think it was necessarily race."

In a statement, Oakland Mayor Libby Schaaf recognized the murder would cause "deep pain," regardless of whether it was motivated by race.

"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.

Oakland residents visit a memorial to Nia Wilson at the BART station where she was killed.

Alameda County District Attorney Nancy O'Malley, whose office is prosecuting Cowell, said her office is still investigating whether the stabbing could be a hate crime, KGO reported.

"There has to be evidence that it was motivated by hate and in this case, whether it was motivated by their gender or by their race, we're still gathering the evidence to look at that," she said Wednesday.

Allums, the godfather, isn't sure if the attack was racially motivated, but it won't change the fact that his goddaughter is gone.

"I don't know if it was racist," he said. "I don't know if it was random. I don't know these crazies. I don't know what it is. I know that my baby is dead. And it's a senseless death."