A Washington state man charged this week with plotting to blow up a Wal-Mart and two gas stations as a diversion for a robbery spree has been found dead in his cell.

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 the death as an apparent suicide, but no details were immediately released about the cause.

Prosecutors say 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.