A bomb threat charge has been dropped against a man accused of placing a backpack with a rice cooker near the Boston Marathon finish line on the anniversary of last year's deadly bombing.

Suffolk County district attorney's spokesman Jake Wark said Tuesday the state could not prove Kevin Edson communicated a clear threat with an explosive device.

Last year's attack involved backpacks with pressure cooker bombs. Three people died.

Edson was spotted with a backpack near the finish line April 15 wearing makeup and a black veil. His mother has said he has battled bipolar disorder. A state psychiatric hospital deemed him competent for trial.

Edson is charged with possession of a hoax device, disturbing the peace, disorderly conduct and disturbing a public assembly. He has pleaded not guilty.