The boat explosion on Lake Lanier that injured three people, including a 13-year-old girl, Sunday was caused by mechanical problems, authorities said.

A family of three was riding in a 1957 Chris Craft around 2 p.m. close to Buford Dam Park when the boat began to experience mechanical problems. After 20 minutes, the boat caught fire and exploded, according to Gwinnett County fire spokesman Lt. Colin Rhoden.

The two adults on the boat were treated for non-life-threatening injuries. The girl was taken to Grady Memorial Hospital with third-degree burns, according to Rhoden.

“The explosion actually knocked (the girl) out of the boat and into the water,” Joe Jordan, a witness, told Channel 2 Action News on Sunday. “It sounded like a bomb had gone off.”

The boat was not equipped with the equipment to remove flammable gas fumes from the engine, which Gwinnett County arson investigators believe caused the fire, Rhoden said.

“Although the exact ignition source was undetermined from several possible sources, the incident with the explosion and resulting fire was determined to be accidental,” Rhoden said in a press release.