BOSTON (AP) _ A tanker truck exploded Sunday on Interstate 93, sending a ``tower of flames'' shooting into the air and closing the highway in both directions.

Four people suffered minor injuries.

Massachusetts State Police Lt. Paul Maloney said it was not immediately clear what caused the blast on the crowded highway in the city's Dorchester section.

Tom Lyons of Boston Emergency Medical Services said the tanker driver sustained minor injuries. Three others were transported to a hospital. One had an ankle injury, one had a shoulder injury and a firefighter hurt his knee.

Initial reports indicated the truck was loaded with gasoline.

``One guy got out of a car and ran. He was limping, he was hurt,'' said Bill Hickey, a steward at the nearby Dorchester Yacht Club and a witness.

Hickey said he ran outside the yacht club after hearing a loud explosion.

``It was just a tower of flames,'' he said.