Gunmen in Northern California fired shots into a moving party bus early Saturday, wounding four and leaving authorities stumped because passengers refused to cooperate with the investigation.

The chartered executive limo bus was leaving San Francisco and had exited the Bay Bridge around 2:25 a.m. when two gunmen in a red SUV drove alongside and opened fire, said California Highway Patrol Officer Daniel Hill.

A fourth passenger sustained was hurt critically but was stable and is expected to survive. Three others had minor injuries.

Interstate 880 was closed for roughly five hours as officers investigated.

Passengers adamantly refused to give witness statements or provide descriptions of the shooters, authorities said.

"In fact, they didn't cooperate so much that one of our officers got assaulted and kicked by one of the victims," Hill said.

A female victim hit by a ricochet also got into an altercation with emergency medical personnel and forced her way out of an ambulance. She fled and her whereabouts are unknown.

Investigators say the bus was occupied by couples but Hill could not provide an estimate of how many people were on the bus. He declined to release names and ages of the wounded.

"I will say we don't believe this is a random incident," he said.

The Oakland Tribune reports the incident is the eighth freeway shooting in the East Bay since November. Officers have blamed the shootings on an escalating gang war in the Richmond area where six of the shootings have taken place.

Freeways are appealing because there are fewer surveillance cameras and easy getaways.

Hill said the CHP does not believe Saturday's shootout is specifically related to the other freeway shootings.