An autopsy determined that a Washington State Penitentiary inmate died March 18 from a pulmonary embolism, said Walla Walla County Coroner Richard Greenwood.
Michael Chadd Boysen, 28, lost consciousness when he returned to his cell at the Walla Walla prison after taking a shower, according to the Associated Press. He was pronounced dead after prison staff tried to revive him.
Boysen had served 16 months of a life sentence without the possibility of parole for two first-degree murder convictions in King County. Boysen strangled and robbed his grandparents at their Renton home in 2013.
Robert Taylor, 82, and Norma Taylor, 80, were killed after they picked up their grandson following his completion of a previous prison sentence for robbery.