A 26-year-old man is accused of killing his grandparents and setting their Key Peninsula house on fire.

Firefighters were called about 8:30 p.m Sunday to a home in the 18800 block of Bayview Road Northwest and found the house completely engulfed in flames.

Witnesses reporting hearing an explosion in the house.

Crews found two bodies in the basement, later identified as a 71-year-old man and his 73-year-old wife.

Their names have not yet been released.

Investigators immediately suspected the deaths were homicides and the fire an arson because the couple’s grandson, who also lived there, and a vehicle were missing.

The family’s missing vehicle was found about 4 a.m. Monday outside a Lakewood apartment complex in the 6600 block of 150th Street Southwest.

The mother of grandson’s girlfriend lives there, sheriff’s spokesman Ed Troyer said.

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A SWAT team responded, and the grandson was taken into custody about 7:30 a.m.

He was booked into Pierce County Jail on two counts of first-degree murder and one count of first-degree arson.

A motive has not been determined for the killings.

Investigators were on the scene of the Vaughn, Pierce County, fire Monday.