(CLAY TOWNSHIP) -- Firefighters have mostly extinguished a grass fire that spread across a marshy part of Harsens Island in Lake St. Clair. Clay Township officials say firefighters from multiple agencies were tackling the Thursday afternoon blaze that sent up plumes of thick black smoke. The fire was reported about 3:30 p.m. and was mostly out by 6 p.m. Assistant fire chief Bob White says a cause is unknown. There are no reports of injuries or damaged buildings. A ferry offers the only access to the island at the mouth of the St. Clair River in St. Clair County.