Investigators have found no physical evidence of an explosion on the ground after a woman on a Rhode Island beach was thrown into an rock jetty, and officials said Monday there may never be an explanation as to what exactly happened.

"It could be a lot of different things," state police Col. Steven G. O'Donnell said. "We may not have a definitive answer."

Police and the state's fire marshal are investigating whether there was an aerial explosion or an explosion below ground at Salty Brine beach in Narragansett, O'Donnell said. Investigators are also looking into rock shifts and seismic events, he said.

"You could have an aerial explosion that could create a boom. Anything on ground, we've ruled that out," O'Donnell said.

Kathleen Danise, 60, of Waterbury, Connecticut, was thrown 10 feet from her beach chair into the rock jetty on Saturday, according to Danise's family members. Witnesses told police they heard a loud bang.

Danise suffered two fractured ribs and bruising, her family said. She was released from the hospital on Sunday.

Police never theorized that the event was caused by a grenade or another explosive device, O'Donnell said.

"You never rule anything out, but you go where the evidence takes you," O'Donnell said. "We don't have evidence that it was anything on the ground at this point."

He said there's typically physical evidence with an explosion, such as charring or debris, which investigators haven't found.

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