One driver was forced from his BMW at a red light, marched to a curb and killed as witnesses watched in horror.
"He was basically executed," Santa Ana police Cpl. Anthony Bertagna said. "There were at least six witnesses."
The shooter, Ali Syed, was an unemployed, part-time student who lived at the Ladera Ranch residence where the first victim was slain, Tustin police Chief Scott Jordan said.
Orange County sheriff's spokesman Jim Amormino said the woman killed at the home was in her 20s. She was not identified and is not related to the shooter, he said.
Jordan said Syed stated to one carjacking victim: "I don't want to hurt you. I killed somebody. Today is my last day."
The violence began at 4:45 a.m., when deputies responded to a call in Ladera Ranch, a sleepy inland town about 55 miles southeast of Los Angeles. They found the woman shot multiple times.
Jason Glass, who lives across the street, said he couldn't sleep and was watching TV in his garage with the door partly open when he heard what sounded like gun shots.
Then he heard a commotion and the sound of a car speeding away.
Hours later, his neighborhood was flooded with police, and crime scene tape sectioned off the street.
Glass said a man and three young children had been escorted from the home where the shooting occurred.
"I just happened to be in here when this happened," Glass said about his garage. "To think he could have rolled under my door or needed a car or needed to hide is crazy. It's freaking me out."
From Ladera Ranch, police said the gunman headed north and within 30 minutes carjacked a Dodge pickup truck in Tustin, about 20 miles away. The driver was uninjured, but a bystander was hit by gunfire and taken to a hospital.
The suspect then began firing at vehicles in the area where Interstate 5 and State Route 55 connect.
Three people reported being targeted, including one who suffered a minor injury, Tustin police Lt. Paul Garaven said. Two cars were damaged.
When the truck got low on gas, the gunman stopped at State Route 55 and McFadden Avenue in Santa Ana, stole the BMW and killed the driver, Bertagna said.
The shooter then drove to a Tustin business called Micro Center and carjacked another small truck, killing one person and wounding another, Garaven said.
Officers trailed the gunman to Orange, a city about five miles away.
As they closed in, the man got out of the vehicle at a busy intersection and shot himself, police said.
A shotgun was recovered at the scene.