OAKLAND, Calif. — Someone opened fire on a party bus on a San Francisco Bay Area freeway early Tuesday, killing two women and wounding at least three others, authorities said.

The passengers were celebrating a woman’s 21st birthday when the bus was attacked around 12:20 a.m. Tuesday on Interstate 580 as it was returning to Oakland from San Francisco, the East Bay Times reported.

A shooter or shooters in another vehicle began firing at the bus and then followed it off the freeway and continued shooting at it on city streets, authorities said. Dozens of rounds were fired at the party bus.

“Based on preliminary investigation, it does not appear this shooting was a random event,” according to the California Highway Patrol, which is investigating along with Oakland police.

At least three women were wounded and taken by ambulance to a hospital from an Oakland police station, where the bus driver drove after the shooting.

One of the women killed was a 19-year-old Stockton resident, relatives told the newspaper. The Alameda County Coroner’s Bureau said one woman died on the bus and the other at an Oakland hospital.

A man who identified himself as the father of the 21-year-old celebrating her birthday said she was shot in the leg but is in stable condition.

Last month, a 25-year-old woman was shot while riding on a party bus in Oakland. Authorities said there were more than 30 passengers on the bus that was supposed to go to San Francisco. None of the other passengers or the driver were injured, and the woman survived.