Investigators say there may never be an explanation about what happened on a Rhode Island beach when a mysterious boom was heard and a woman was thrown into a rock jetty.

State police Col. Steven G. O'Donnell said Monday that investigators have found no physical evidence of an explosion at Salty Brine beach in Narragansett.

O'Donnell says the state's fire marshal is investigating other possible causes, like seismic events.

Witnesses say 60-year-old Kathleen Danise was thrown 10 feet into a rock jetty Saturday after a loud bang. Danise's family says she suffered two fractured ribs and bruising. She was released from the hospital Sunday and returned to her home in Waterbury, Connecticut.

The beach was reopened Sunday after officials found no evidence of an explosive device or gas line under the sand.