A 20-year-old student shot and killed a woman in her home and two commuters during carjackings, shot up vehicles on a Southern California freeway and committed suicide as police closed in on him Tuesday, authorities said.

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 Orange County 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 killings happened not long after a bloody saga that gripped Southern California for six days, as former Los Angeles police officer Christopher Dorner eluded a massive manhunt after killing three people. Dorner died Feb. 12 from an apparent self-inflicted gunshot wound to the head after a fiery gun battle that left one police officer dead.

The violence began at 4:45 a.m. local time, when deputies responded to a call in Ladera Ranch, a sleepy inland town southeast of Los Angeles. They found the woman shot multiple times.

After the shooting, a neighbor said a man and three young children were escorted from the home where the shots were fired.

The gunman then headed north and within 30 minutes carjacked a Dodge pickup truck in Tustin, about 20 miles (32 kilometers) away, police said. The driver was uninjured, but a bystander was hit by gunfire and taken to a hospital.

The suspect then began firing at vehicles in a busy intersection of highways.

Three people reported being targeted, including one who suffered a minor injury, Tustin police Lt. Paul Garaven said. Two cars were damaged.

When the gunman's truck got low on gas, he stopped 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 (8 kilometers) 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.

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Associated Press Writer Sue Manning in Los Angeles contributed to this report.