Family of Francisco Serna say he was suffering from dementia and was shot nine times, in same county found to have highest per capita rate of killings by police

A Bakersfield police officer shot and killed an unarmed 73-year-old man on Monday. Family members said Francisco Serna was suffering from dementia and was shot nine times as he took a walk outside his home in the early hours of the morning.

Bakersfield lies in Kern County, California, where a Guardian investigation last year revealed that law enforcement officers killed more people per capita than any other county in the US. The Bakersfield police department and the Kern County sheriff’s department are the two largest law enforcement agencies in the county.

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Police said in a statement that officers had received a report of a man armed with a gun in Serna’s neighborhood. When officers arrived in the area after 12.30am on Monday, a witness pointed to Serna standing in a driveway before an officer fatally shot him, according to the police account.

The Bakersfield police spokesman, Sgt Gary Carruesco, later confirmed to the Guardian that Serna did not have a firearm and no weapon was recovered from the scene. Carruesco said the department expected to release the name of the officer who opened fire later on Tuesday.

Francisco Serna. Photograph: Facebook

Serna’s son Rogelio Serna, said in a Facebook post that his father had been “murdered” by police and the family wanted “the truth to be told”.

“Right across the street is where the police shot my father with nine bullets to his body,” Serna’s son said in a video on Facebook. “My dad was not armed … there’s false reports about my father having weapons, he was never armed.”

Serna also posted photos of his father and and message about his “last walk”. Rogelio Serna said his father was in the “early stages” of dementia and that he took walks when he had difficulties going to sleep. Early on Monday morning, Rogelio wrote: “HE TOOK HIS LAST WALK,AND WAS KILLED IN A 15 MIN WINDOW FRAME RITE [sic] ACROSS THE STREET FROM HIS HOME.”

The family plans to hold a vigil on Tuesday evening.

In 2015, 14 people were killed by police in Kern County, which has a population of just under 875,000. In the same time frame, 10 people were killed by the New York police department throughout the five counties of New York City, home to nearly 10 times as many people and about 23 times as many law enforcement officers. In 2016, the rate of people killed by police has dropped significantly in the county; Serna was the fifth person to be killed by law enforcement officers in the county this year, according to The Counted, the Guardian’s ongoing investigation into officer-involved deaths throughout the US.

The shooting will be subject to an internal review by Bakersfield police department’s critical incident review board, which is composed of a panel of senior officers. This internal investigation will then be referred to the Kern County district attorney’s office for review. In 2015 the Guardian revealed that the vast majority of investigators working in that office were former law enforcement officers at departments in the county.

Following publication of the Guardian’s investigation a number of prominent local advocates called on the US justice department to investigate the Bakersfield police department.