DALLAS -- Authorities are continuing their investigations into rush hour shootings that left two drivers dead and a third injured on roads in northeast Dallas and Garland on Monday.

“I actually think this is a random act,” Lt. Craig Miller of the Dallas Police Department said at a news conference that began shortly before noon. “Someone was upset… I don’t see this as a long-term problem.”

Dallas police said they were working with Garland police to determine whether the incidents in the two cities were related and would compare ballistics information.

Dallas police are also encouraging any witnesses to come forward. Garland police have described the shooter in their incident as a balding white man in his 40s driving an extended-cab Ford F150 pickup.

Investigators were also trying to establish whether there was any connection between the road shootings and a standoff later with driver in a Honda that ended in a suicide attempt.

The first fatal shooting was reported about 5:40 p.m. in Garland just north of LBJ Freeway. Jorge Lopez of Rowlett was driving a small two-door Nissan on Jupiter Road when he stopped at a red light at Marquis Drive, Garland police said. A Ford pickup pulled up alongside his car and a man fired several shots at Mr. Lopez, killing the 20-year-old man, said Garland police Officer Joe Harn.

Police said the pickup fled south on Jupiter Road. Minutes later it was reported that a man began shooting at a semi with no trailer on LBJ between Royal Lane and Skillman Street. No one was injured.

Police said a driver then shot at a 42-year-old man driving a United Van Lines 18-wheeler. The trucker, William Scott Miller, 42, of Frankfort, Ky., was killed. His rig came to a stop in the middle lane of westbound LBJ just east of the Miller Road exit. Mr. Miller was about to park his rig.

Mr. Miller was planning to fly home to be with his wife and children, Dallas police said.

Lt. Miller called the truck driver's actions "heroic." No other people were injured as Mr. Miller maneuvered his truck out of harm's way. "Even though he was mortally wounded, he was able to control his rig," Lt. Miller said.

Farther west on LBJ near Skillman Street, another truck was struck by bullets — the fourth incident. That driver was struck by flying glass, said Jesse Medford, terminal manager with Dugan Truck Line. The driver was taken to an area hospital.

Mr. Medford said the driver, his employee, radioed him saying, “I’m getting shot at! I think I’m shot!” The trucker said he didn’t know why he was being fired upon, Mr. Medford said.

Mr. Medford told his driver to pull over, then Mr. Medford called 911.

Mr. Medford did not identify his driver other than to say he’s a longtime trucker in his 40s. He said the driver told him that he did not know who was shooting and did not give a detailed description of the assailant.

“He didn’t say anything about any type of road rage,” Mr. Medford said.

Rush hour traffic slowed to a crawl along the highway as police shut down all westbound lanes of LBJ east of the homicide scene for several hours. Drivers used access roads and alternate routes to get home as authorities began an intense investigation at the site.

In the later shooting incident, a man who had been accused of a robbery in Keller was listed in critical condition after he shot himself during a SWAT standoff.

Keller police had issued robbery and burglary warrants for Brian Smith, 37, who authorities believe is a former Utah state trooper. Bulletins issued by Keller police said Mr. Smith was believed to be driving a dark-colored Honda and described him as armed and possibly suicidal.

A Garland police officer spotted the Honda, with Mr. Smith inside, about 9 p.m. Monday on State Highway 66 at Commerce Street in Garland. Mr. Smith did not respond to the officer’s commands, and the SWAT team was called in, Garland police said.

Shortly after midnight, Mr. Smith put his vehicle in gear and started to drive. Police blocked his car with their vehicles. When the SWAT team approached Mr. Smith’s vehicle, officers heard a gunshot from inside the Honda.

Mr. Smith was injured with what appeared to be a self-inflicted gunshot wound, Garland police said. He was taken to Parkland Memorial Hospital.

Schepp’s Dairy is offering a $10,000 reward for any information leading to the arrest or conviction of the man who killed the two drivers. Anyone with information can call (214) 671-3661.

The Associated Press contributed to this report.