Goyette had faced 64 counts of threats and hoaxes over last fall's letters.
Nobody was hurt. The powder was harmless.
The latest news from around North Texas.
Goyette faces up to 15 years in prison, a $500,000 fine and restitution. Sentencing is May 25.
An attorney for Goyette didn't immediately comment.
Officials have said Goyette was apparently upset about losing more than $60,000 in Washington Mutual Bank stock he held when the feds took it over in September.
The next day the FDIC sold the bank's deposits, branches and loan portfolio to JP Morgan Chase for a fraction of their combined value.