The cause of a fire that seriously damaged a home at 3208 Gratiot Ave. in Port Huron remains under investigation.  Five units from Port Huron, Port Huron Township and Marysville attended the fire, which was reported around 6:20 p.m. Friday.  Neighbors in the area reported hearing what may have been an explosion at that time.   Fort Gratiot firefighters provided backup at the Port Huron stations.  Two people who were in the house when the fire started were able to escape without injury. The fire was declared under control at about 8:50 p.m. Gratiot Avenue remained blocked until 9:40 p.m.  Reports say the home suffered fire, smoke and water damage