Brian Smith, who shot himself in the head during a standoff with police in Garland, Texas, Tuesday, died about 6 p.m. at Parkland Memorial Hospital in Dallas, the Fort Worth Star-Telegram reported.
Meanwhile, Dallas police said they closed a section of a highway Wednesday to search for more evidence in the shootings.
The Dallas Morning News reported city police homicide detectives said ballistics tests had linked Smith to the killing of truck driver William Scott Miller, but police in the suburb of Garland said they need more tests before identifying him as the killer of Jorge Lopez.
Miller, 42, and Lopez, 20, were fatally wounded on the LBJ Freeway during evening rush hour Monday. Another driver was wounded by someone firing from a pickup truck.
Smith shot himself after Garland police pulled him over.
Smith, a 12-year veteran with the Utah Department of Public Safety and father of five, was struggling with an addiction to painkillers following an on-duty accident, the News said. Smith was forced to resign in May after an internal investigation found he drove under the influence, committed theft and was engaged in conduct that would "jeopardize public trust and fidelity in law enforcement."