Authorities in Oakland on Monday arrested a 27-year-old man in connection with a fatal knife attack on an 18-year-old woman and her sister at a train station on Sunday night. The suspect, identified as John Cowell, is accused of stabbing two sisters as they transferred trains at MacArthur Station. Nia Wilson died after being stabbed in the neck, while her older sister, Lahtifa Wilson, was wounded. Nia Wilson reportedly died on the train platform calling out for her sister, who stayed with her trying to comfort her. “It looks like it was an unprovoked, unwarranted, vicious attack,” Bay Area Rapid Transit Police Chief Carlos Rojas said Monday. The killing was the third possible homicide on the BART line in less than a week, though authorities have said they don’t believe the three are related. A 47-year-old man was punched at another train station on Saturday and died after falling on the track and striking his head. Another man died Friday after an altercation at a train station earlier in the week.