Police said Tuesday they were investigating several "persons of interest" in four rush-hour shootings on Dallas-area roads that left two people dead, and hailed one of the victims as a hero for managing to stop his 18-wheeler before he died.

Dallas police have interviewed at least two people who witnessed one of the apparently random Monday afternoon shootings and were asking any other witnesses to step forward.

Witnesses described the suspect in the first shooting, in the suburb of Garland, as a balding white man in his 40s, driving a tan Ford F-150 extended cab pickup. Police said they did not know whether one person was responsible for all the shootings.

The first shooting happened when the pickup driver pulled up alongside a small Nissan stopped at a red light and began shooting, Garland police spokesman Joe Harn said. The Nissan's driver, 20-year-old Jorge Lopez of Rowlett, was killed.

Witnesses told police the pickup then drove off toward Interstate 635 in Dallas, where shots were fired at an 18-wheeler a short time later. The driver of the 18-wheeler, identified by police as Kenneth Black Harly, was not hurt.

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."

United Van Lines posted a statement to its Web site Tuesday that said Miller had a flawless safety record and was "an asset to our company both personally and professionally."

"We are deeply saddened by this senseless act," the statement said.

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."

"It's just absolutely stunning to me that something like this would happen," Wilhite said. "This is our way of surviving in this country — truckers hauling goods up and down the highways. Why would someone want to take potshots like this at our drivers?"

While he was being shot at, Roberts radioed his employer to tell them what was happening. Jesse Medford, a Dugan terminal manager, told The Dallas Morning News that Roberts yelled: "I'm getting shot at! I think I'm shot!"

After telling Roberts to pull over, Medford said, he called 911. Roberts told him that he did not know who was shooting and could not give a detailed description of the assailant.

Lt. Miller said he believes the suspect was angry and that there was no pattern in selecting the victims.

"I think this is a very random act," Miller said. "I don't see this as a long-term problem."

Police said they had no description of the weapon used in the shootings but expected to know more after analyzing bullet fragments.

Police also said they would investigate whether there was a connection between the shootings and a former Utah state trooper wanted on burglary and robbery warrants who apparently shot himself early Tuesday morning during a standoff with police in the Dallas suburb of Garland. He is in critical condition at a local hospital.

Click here for more from MyFOXDFW.com.