SAN QUENTIN (CBS13) – A San Quentin death row inmate from Contra Costa County was found dead in his cell late Sunday evening.

Forty-one-year-old Justin Alan Helzer’s death at the state prison is being investigated as a suicide. He was single-celled.

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Helzer was sentenced to death, along with his brother, Glenn Helzer, on March 11, 2005 by a Contra Costa County jury for killing five people in an extortion and murder scheme.

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Since the reinstatement of California’s capital punishment in 1978, 58 inmates have died from natural causes, 22 have committed suicide, 13 have been executed in California — one was executed in Missouri — and six have died from other causes.

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There are 733 offenders on death row in California.