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 released images last week of the unidentified suspect in the attack. Monday, it said the images, captured aboard a train and an AC Transit bus, depicted Combs.
BART police had already made arrests in the two other fatal incidents involving the agency.
Aided by tips from passengers, BART police arrested John Lee Cowell last Monday for the knife attack that killed Nia Wilson and injured her sister, Lahtifa, 26, the previous evening. Cowell has been charged with murder.
In the third incident, BART reported that Gerald Busbee, 51, of Pittsburg, died July 20, two days after suffering what appeared to be minor injuries in an altercation at the Pleasant Hill Station with a fellow passenger, Abdul Bey, 20.
Chief Rojas said that Busbee was injured July 18 in a fight 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.