Hamas security arrested dozens of supporters of the rival Fatah group early Saturday, Fatah said, following a mysterious beachside blast that killed five Hamas members and a 6-year-old girl. The cause of the blast remained unclear, though the circumstances suggested it was not set off inadvertently by terrorists mishandling explosives. The explosion went off late Friday in a car parked near a crowded Gaza City beach. Hamas said that at the time, a group of Hamas gunmen was picnicking on the beach. Three Hamas members, including a field commander, and the girl were killed immediately. Hospital officials said two more Hamas members died of injuries Saturday. Fifteen people were wounded.