An early morning explosion that wrecked a post office collection box in Woodacre, perhaps as a graduation prank gone awry, has launched a full federal investigation and up to a $10,000 reward for information leading to an arrest and conviction. Neighbors reported hearing four or five loud bangs around 3:30 a.m. on Sunday, first suspected to be gunshots. The blast was determined to have come from a large firecracker, said Lt. Jamie Scardina, with the investigations division of the Marin County Sheriff’s Office. There were no injuries and no suspects as of Tuesday afternoon, he said. The bottom of the blue steel receptacle was completely blown out, and mail was strewn across the area. Almost all the mail was recovered and sent on, but a few damaged pieces are being held as evidence, said Jeff Fitch, a federal agent and spokesman for the Postal Inspection Service, the law enforcement arm of the U.S. Mail. Intended recipients and senders whose mail is undelivered will be contacted, he added. A conviction for damage to the collection box and mail could result in fines up to $1,000 and up to three years in prison. Anyone with information can call the postal inspector’s 24-hour tip line at (877) 876.2455.