An out-of-control truck veered into a guardrail of the Coronado bridge in San Diego on Saturday, causing it to plummet into a park below, where revellers were enjoying the Chicano Park music festival.

The impact of the pickup truck, which fell about 60ft (18.3m) according to reports, killed four people almost instantly. Two of the nine people injured are said to have suffered major trauma.

The driver, who has been identified as off-duty Navy serviceman Richard Anthony Sepolio, 25, was taken to the University of California San Diego hospital with major injuries and later arrested on suspicion of driving under the influence causing injury or death.

Sepolio is stationed at Coronado Island, which is accessed via the bridge from which the driver plummeted.

The victims have not yet been named, but California Highway Patrol said they were a 62-year-old man and 50-year-old woman from Arizona, and a 59-year-old man and 49-year-old woman from Hacienda Heights, in the Los Angeles area.

Witnesses at the scene told reporters two of the victims were vendors at the festival in the Bario Logan area of the city, which is a predominantly Mexican-American neighbourhood.

There were around 100 people in the park at the time of the incident at an event held alongside the La Raza Run motorcycle festival, police said, describing the aftermath of the crash as horrific.

Speaking at the scene, California Highway Patrol officer Jake Sanchez told reporters: Its horrible, its horrific, said Sanchez, describing the scene. Theres people down below, bodies – just innocent people that are just down here having a good time and now theyre gone. We feel for their families.

Asked about the specifics of the crash, Sanchez said he had no information about the reason for the crash, of a chase or exact speed. Though he did confirm alcohol was involved.

A witness at the park, Dolores DAngelo told local news channel San Diego 7: Pieces of car parts were flying all over the place. When we got up, we turned around, everybody started running over there because we realised, there were people sitting there – there were people sitting right there.

We were trying to get in to see if we could maybe lift up the car, move the car out of the way to see if we could get the people out. There were so many people crowded by then.

The Mayor of San Diego, Kevin Faulconer tweeted on Saturday: Praying for victims and families affected by the tragedy in Chicano Park. Devastated this occurred in one of our communitys most beloved places