SAN DIEGO, Calif. - Authorities in San Diego were trying to determine Sunday why the driver of a pickup lost control of his truck on a highway bridge, swerved over a retaining wall and plunged 60 feet into a park, killing four people at a biker festival.

CAUGHT ON VIDEO: Moments after San Diego crash

The Medical Examiner identified the deceased as 52-year-old Cruz Elias Contreras and 50-year-old Annamarie Contreras from Chandler, Arizona, and 59-year-old Andre Christopher Banks and 49-year-old Francine Denise Jimenez from Hacienda Heights, a suburb east of Los Angeles.

Police said Richard Anthony Sepolio, a Navy member stationed at Naval Base Coronado, was arrested on suspicion of driving under the influence causing death.

"It's horrible, it's horrific," California Highway Patrol Officer Jake Sanchez said late Saturday. "There were people down below. ... Innocent people that were just down there having a good time and now they are gone."

Sanchez said the four people who died were killed almost instantly. Sepolio, 25, and one person on the ground in Chicano Park survived with "major" injuries, Sanchez said. Seven other people also were injured.

"Praying for victims & families affected by the tragedy in Chicano Park. Devastated this occurred in one of our community's most beloved places," Mayor Kevin Faulconer tweeted.

The festival marked La Raza Run, an annual motorcycle ride from Los Angeles to San Diego that can draw more than 1,000 bikers. The festival features food, music, art and tables of vendors selling shirts and other goods.

Victims described the scene as the truck crashed onto a vendor's table where people had crowded around to buy and sell T-shirts.

Dolores D'Angelo told has been singing with a band at the festival when she witnessed tragedy and chaos.

"I saw a car flying through the air, right off the bridge," she said. "It looked like as if Superman had thrown a car right off the bridge. It was just flying down."