Four people, including a child, have been injured after a boat exploded at a southwestern Michigan marina.

Authorities say the force of the blast knocked the victims into the water about 2 p.m. Sunday at a marina in South Haven, about 50 miles southwest of Grand Rapids.

The victims had to be rescued from the water.

Authorities say at least three suffered burns. They were being treated at a Kalamazoo hospital.

The cause of the explosion is under investigation, but it started a fire that spread to several other boats.

Damages could exceed more than $500,000.