The rampage began just as the morning commute was getting underway.

Over the course of 75 terrifying minutes, a 20-year-old college student allegedly killed a woman at his Ladera Ranch home before embarking on a string of shootings that stretched through the heart of Orange County and targeted random people during their morning routines.

He carjacked a truck at a gas station, police said. He then allegedly executed a businessman and stole his BMW. A few minutes later, he killed a plumber and took a work truck. He shot indiscriminately at morning commuters on the 55 Freeway, hitting at least three cars.

PHOTOS: Shootings at multiple locations in O.C.

In the end, authorities say he killed three people and wounded three more before turning his gun on himself.

Authorities were stunned by what they described as the “senseless violence,” which spanned about 25 miles of normally placid suburbia. The gunman, identified as Ali Syed, had no criminal record and left few clues as to his motive, police said.

“I just killed someone,” Syed told one man during the shooting spree, according to police. “This is my last day.”

MAP: Orange County shootings

Police discovered the gunman’s first victim early Tuesday at a beige-and-white stucco condo in an upscale Ladera Ranch community. Syed, who was unemployed and enrolled in a single class at Saddleback College, lived there with his parents.

Neighbor Jason Glass said he heard three to five loud bangs early Tuesday morning.

He heard doors slamming. Then a car vroomed away from the house.

MAP: Orange County shootings

“I just thought somebody was being really loud and obnoxious,” he said.

Deputies arrived at the neighborhood of doctors and lawyers about 4:45 a.m., after Syed’s parents called 911.

Authorities found the body of a woman in her 20s who’d been shot multiple times. She was not related to Syed, authorities said. As of Tuesday evening, it remained unclear who she was and why she was at his home.

Meanwhile, Syed had taken off in the family’s black GMC Yukon, armed with at least one shotgun. Possibly in his haste to flee, police said, his SUV sustained some damage.

He exited the 5 Freeway at Red Hill Avenue in Tustin around 5 a.m. and pulled into a Denny’s parking lot. There, a man was sitting in an older-model blue Cadillac, waiting for his son. They’d planned to carpool to work.

Syed, police said, pointed his weapon at the man and yelled: Get out. The man sped away. Syed fired, shattering the Cadillac’s rear window and striking the man in the back of the head. The man fled -– he was later treated at a hospital -– and Syed dashed to a nearby Mobil station, authorities said. He spotted a man pumping gas.