Photo: The Canadian Press. All rights reserved.

A body lays in the intersection of Wanda Road and Katella Avenue early Tuesday, Feb. 19, 2013 in Orange County, Calif., as local police agencies investigate the scene. (AP Photo/The Orange County Register, Mark Rightmire)

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."