Authorities are investigating the source of a possible blast at a Rhode Island beach yesterday that left one woman injured and forced the evacuation of beach­goers on a busy weekend day.

Salty Brine State Beach in Narragansett was evacuated about noon after a woman was knocked into a rock wall by what beachgoers described as a possible explosion.

“It was a weird, deep noise,” David Dean, who was on the beach about 50 feet from the incident, told the Herald. “The next thing I heard was someone yelling, ‘Call 911! Call 911!’ ”

Instead of running away, people were running to help a woman who was in clear distress.

“People were running toward her. I thought she had fallen off the rocks,” said Dean of Woonsocket, R.I., a local radio host. “I still, to be honest, don’t know what the scenario is.”

Dean knew his friends would be headed to the area, and posted a video of the aftermath to his Facebook page with the caption: “Please share! I’m not sure exactly what happened. Do not go to Salty Brine beach today.”

The unidentified 50-year-old woman was taken to a local hospital. Her condition was unknown. No other injuries were reported.

Another witness who was closer to the incident, Mario Lewis of West Greenwich, R.I., told the Providence Journal there was a loud noise and said he saw the woman get knocked over. “It was like an M-80 boom, like a grenade,” he said.

The Rhode Island Department of Environmental Management closed down the beach, and called in the state bomb squad to investigate.

DEM Associate Director Larry Mouradjian told the Herald they excavated and removed all the sand in the area between the high tide and low tide mark where the woman was injured and that material is being tested. He said there was no evidence of a device that caused the noise.

“At this point, we’re not aware of any evidence that suggests a mechanical device,” Mouradjian said. “That’s the understanding I have from the Rhode Island Bomb Squad and the Rhode Island Fire Marshal’s Office. So, once again, there is evidence that something did occur — obviously we have an injured beach patron — but ultimately it is up to the bomb squad and the fire marshal’s office to continue the investigation and determine exactly what happened.”