"We feel safe in saying (Smith) ... was the shooter," Miller said.
Dallas police declined to comment on a second death in neighboring Garland, where the standoff took place, because it was out of their jurisdiction.
Garland police spokesman Joe Harn said his department has not been able to make a definitive connection between Smith and the killing there, but acknowledged that he fit the description of the highway shooter: a balding, 40ish white man.
"We certainly hope it is him," Harn said. "But we are going to have to see more concrete evidence."
Two people were shot and killed and another person was injured by broken glass in four shootings along or near a Dallas-area highway Monday evening. Police believe the victims were selected at random.
Smith was in critical condition Tuesday night at a Dallas hospital from a self-inflicted gunshot wound. Police said he shot himself in the head early Tuesday morning after a brief standoff more than six hours after the shooting spree ended.
Smith had been a Utah state trooper since 1996 but retired in May because of "personal issues," said Sgt. Jeff Nigbur, a spokesman for the Utah Department of Public Safety.
The crime spree appears to have begun in Garland, where a man police identified as Smith jumped over a pharmacy counter at a grocery store and stole Oxycontin pills.
Minutes later, the first shooting happened in Garland when a driver pulled up alongside a small Nissan stopped at a red light and began shooting, Harn said. The Nissan's driver, 20-year-old Jorge "George" Lopez of Rowlett, was killed.
Minutes later on the same highway, a gunman shot and killed 42-year-old William Scott Miller, the driver of a United Van Lines rig, police Lt. Craig Miller said. Police said the driver, who was about to fly home to his wife and two young daughters in Frankfort, Ky., for the holidays, was able to bring his truck safely to a stop before he died.
"The act he did in and of itself I consider to be heroic," Lt. Miller said. "Despite being mortally wounded, he was able to control his rig to the point where other drivers weren't injured."
After the shooting of Miller, another semitrailer was fired upon a half-mile away on the same interstate. The driver, 46-year-old Gary Roberts, was injured by debris and glass but not struck by any bullets. His right eye was hit by shattered glass and he needed several stitches in his fingers, said Bedford Wilhite, who works with Roberts at Dugan Truck Line.
Roberts is home recuperating after being treated at a hospital, Wilhite said. Roberts, who has worked for Dugan for about a year, told Wilhite he is "much blessed and thankful to be alive."
Associated Press writers Regina L. Burns, Andre Coe and Anabelle Garay in Dallas contributed to this report.