A 33-year-old San Marcos woman was arrested on suspicion of felony drunken driving and vehicular manslaughter after the car she was driving early Friday drifted onto the shoulder of Highway 78 and slammed into two women and a stopped car, killing three people, the California Highway Patrol reported.

Just after 12:30 a.m., Debbie Sumi was driving a 1994 Jeep Cherokee when it hit a 2008 Nissan Altima that had stopped on the north shoulder of the freeway, west of Sycamore Avenue, the CHP said.

The Altima's driver had pulled over because a passenger was feeling ill, authorities said.

Susana Orozco, 23, of Escondido, and Erica M. Oliva, 23, of Fallbrook, were outside the car when the Jeep struck them and the Altima from behind, according to the medical examiner's office.

Orozco and Oliva were pronounced dead at the scene, according to the medical examiner's office.

Omar Guzman, 25, of Oceanside, was also outside of the car, but he was standing apart from the women and was not hurt, CHP officers said.

The medical examiner's office said Orozco and Oliva were Valley View Casino employees.

A casino spokeswoman did not return calls for comment Friday. Security personnel later in the day said casino officials had no comment about the incident.

Gerardo Cortez, 21, of Oceanside, was asleep in the Altima's front passenger seat when the car was struck, authorities said. He was ejected from the car, but suffered only a minor injury, the CHP said.

Sgt. Jerry Matheson, a CHP officer who responded to the collision, said it appeared that Cortez had been wearing his seat belt. He said it seemed Cortez was thrown despite the restraint because the Jeep's impact pushed the rear of the car forward with such force that it broke his seat.

The passenger in the right front seat inside the Jeep, a 34-year-old man from Orange, was also killed in the crash, according to the medical examiner's office. His name had not been released Friday night pending notification of his family.

The man's body was found partly through the windshield of the overturned Jeep, Matheson said. He said it did not appear that the man had been wearing a seat belt when the Jeep struck the stopped car.

Sumi was taken to Palomar Medical Center in Escondido and is being treated for her injuries. She was arrested while in the hospital and will be booked into jail once she's released from the hospital, the CHP said.

"It's just sad because it's the type of thing that's very preventable," Matheson said of the apparent drunken-driving collision. "It just shouldn't happen."

Part of Highway 78 was shut down for several hours early Friday, reopening just in time for the morning commute.

Staff writers Edward Sifuentes and Morgan Cook contributed to this report.

Call staff writer Chris Nichols at 760-740-5426.