Scott Sippola and Allison Coss are charged with using interstate commerce to make the threat. It was filed Monday as a "felony information," which means a guilty plea is likely.
Sippola and Coss are accused of threatening to sell photos of Stamos unless he paid them $680,000. They were arrested in December at an airport near Marquette. At the time, Stamos' spokesman said there was nothing embarrassing about the images.
Stamos met Coss in Florida in 2004 and attended a party where photos were taken.
Messages seeking comment were left with the prosecutor and defense lawyers Tuesday.