A white police officer who shot dead a black man in a controversial case in Oakland, California, has been found guilty of involuntary manslaughter.

The family of Oscar Grant, the victim, expressed their disappointment with the outcome. They had been hoping for a murder verdict.

Grant, 22, was shot in the back by policeman Johannes Mehserle, 28, while lying on the platform in a railway station on 1 January 2009. Mehserle claimed he had thought he had his Taser in his hand rather than his gun.

The shooting, which was shown on YouTube, led to a riot in Oakland, and there were fears of further trouble if Mehserle had been found not guilty. Police were yesterday deployed in riot gear in case of any outbreaks of violence.

The case has become a cause celebre in the US, with its echoes of the treatment of Rodney King, a black man whose severe beating by police in LA in 1991 was captured on video. The subsequent acquittals of four LAPD officers sparked the 1992 Los Angeles riots.

The verdict means the jury thought Mehserle had been criminally negligent but had not intended to kill Grant.

Mehserle, who is to be sentenced next month, could face anything from five to 14 years in jail.

John Burris, a lawyer representing the family, described it as a "compromise verdict".

"The system is rarely fair when a police officer shoots an African-American male," Burris said. "No true justice has been given."

The trial was held in Los Angeles because of the tension in the Oakland and neighbouring San Francisco over the shooting.