King County Sheriff's detectives say they believe 26-year-old Michael (Chad) Boysen may be seeking to obtain firearms because he has been searching for gun shows on the Internet.
Authorities have accused Boysen of killing his 82-year-old grandfather and 80-year-old grandmother in their Renton home late Friday or early Saturday.
Sheriff's spokeswoman Sgt. Cindi West says their bodies were discovered Saturday. The cause of the deaths hasn't been released.
At first police said he wasn't connected to the deaths, but later he was named a suspect.
He had recently been released from prison after serving a year for burglary.
Police are also searching for the victims' missing red, 2001 Chrysler 300.