The Ionia County Sheriff's Department and the state police fire marshal made the announcement Tuesday.
Investigators found an accelerant on wood samples taken from Whites Bridge on July 8. That's a day after the 146-year-old covered bridge was destroyed by what initially was viewed as a suspicious fire.
The bridge was built in the late 1860s. It crossed the Flat River in Ionia County's Keene Township, about 15 miles east of Grand Rapids.
There's a $7,000 reward for information leading to the arrest and conviction of whoever was responsible.