Lawyers for the family of slain teen Nia Wilson said Wednesday that BART bears responsibility for her death allegedly at the hands of a violent felon because the agency fails to prevent thousands of fare evaders from flooding onto trains every day.

“BART’S failure to take criminals out of the system led to this tragedy,” said Robert Arns, an attorney representing the family of Wilson, 18, of Oakland, who was killed July 22 as she boarded a BART train at MacArthur Station, and her sister, Lahtifa Wilson, 26, whose neck was slashed.

John Lee Cowell, 27, a convicted felon with a history of violent behavior, has been arrested on suspicion of stabbing the sisters in an attack captured on BART video footage.

Arns noted that crime has soared on BART, and asserted that there is an overlap between criminals on trains and fare evaders. It’s not clear if Cowell evaded fares on the night of the killing, but he was cited for fare evasion on July 18.

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The state Department of Justice also released figures last month showing that the number of violent crimes on BART jumped 69 percent in the last decade, even as they fell 7 percent across California.

Arns said the family is preparing to sue BART for monetary damages for the death of Nia Wilson and for the injuries to Lahtifa Wilson. As part of the suit, he said the family will ask that BART be required to take specific steps to “stop the escalation of violent crime, and change the culture of BART.”

According to the lawyer, the family will ask that BART “staff up the turnstiles” so that fare evaders will be caught in the act; provide accurate, up-to-date information about criminal activity throughout the system and at each station; and improve policies and staff to better deter deter crime.

In response to the allegations, BART issued a statement Wednesday expressing condolences to the family of Nia Wilson, defending its efforts to fight fare evasion and improve surveillance.

“In the last several years BART has launched a multi-prong approach to reduce fare evasion, including a new proof-of-payment ordinance and inspection teams, as well as infrastructure changes to make it harder to bypass fare gates,” BART spokeswoman Alicia Trost said in a statement.

She said every train now has “working cameras,” for a total of 4,000 throughout the system.

“We are thankful the suspect (in the Wilson slaying) is in custody due in large part to our surveillance system,” she said.

Cameras were installed inside every BART car after The Chronicle revealed in 2016 that most surveillance cameras in the trains were fake, and that real ones were often broken. This made it impossible for BART to review footage of a passenger killing in January of that year that remains unsolved.

Trost said that arrests “surged last year by nearly 40 percent, due to the fact our officers were in the right place at the right time.” She also defended BART’s transparency in publishing crime data “through multiple venues.”

The agency has been criticized for delaying or failing to tell the public about a number of cases, including two deaths in the days just before Wilson was killed.

On July 21, a man at the Bay Fair BART Station punched Don Stevens, 47, a transient, who fell and struck his head on the platform, police said. Stevens died the next day, and a suspect in the case has been arrested.

On July 18, a man at the Pleasant Hill BART Station attacked Gerald Bisbee, 51, who died two days later from an infection to a cut on his leg, BART later reported. An arrest has also been made in that case.

Meanwhile, Cowell remains in custody, and his motive for allegedly attacking the Wilson sisters is still unknown.

Many in the community — infuriated by the sudden slaying of a young woman riding home on BART like thousands of others — ascribed a racial motive to the killing.

Cowell is white, and Nia and Lahtifa Wilson are black.

What is known is that Cowell has a violent history.

He was convicted of felony second-degree robbery in May 2016, and of being under the influence of a controlled substance two months earlier. Also in 2016, a restraining order was issued against him after staff members at a Kaiser hospital in Richmond said he repeatedly threatened and harassed them.

Another, unspecified, restraining order was filed against him in 2015. He was convicted of battery in 2013, for an incident in Walnut Creek.

At the time of his arrest in July, Cowell was homeless. He is being held without bail in Santa Rita Jail charged with murdering Nia Wilson and attempting to murder her sister. He is due back in court Aug. 22 to enter a plea.

Arns, on behalf of the Wilson sisters’ family, said he doesn’t believe that Cowell paid a fare when he entered BART the day Nia Wilson was killed.

“Do you want to be in a confined space on BART with them?” Arns asked, referring to Cowell and others with a similar history. “It’s time for tough love on this issue.”

Nanette Asimov is a San Francisco Chronicle staff writer. Email: nasimov@sfchronicle.com Twitter: @NanetteAsimov