Tehran, Iran ? An explosion was reported Wednesday in southern Iran near a nuclear power plant, with state-run media offering conflicting explanations for what happened, from blasting for dam construction, a fuel tank dropping from an Iranian plane, and friendly fire.

The explosion came as Iran’s intelligence chief accused the United States of flying spy drones over its nuclear sites and threatened to shoot down the aircraft. But a spokesman for Iran’s Interior Ministry said the blast was heard minutes after an Iranian airplane flew over the southwestern port town of Deylam, about 110 miles north of the nuclear plant, and had not been caused by a hostile attack.

White House press secretary Scott McClellan, asked if there was no U.S. involvement in the incident in Iran, replied: “That’s correct.”