The U.S. Bureau of Prisons said Thursday that 52-year-old Larry Ray Gillette was discovered unresponsive at the Federal Detention Center at SeaTac on Wednesday and was taken to a hospital, but could not be revived.
The FBI is investigating death as an apparent suicide, but no details were immediately released about the cause.
Prosecutors said Gillette was arrested Monday as he tried to ignite what he thought was a car bomb. In reality, it was a dud provided by investigators.
A complaint unsealed Tuesday charged him with soliciting a crime of violence and being a felon in possession of a firearm.