Three U.S. Marines (search) returned fire during Sunday's attack on protesters in the Haitian capital, a Marine major said. It was the first known armed action by U.S. forces sent to stabilize the country.

The Marines reacted when gunmen shot at thousands of unarmed demonstrators who were demanding that ousted President Jean-Bertrand Aristide (search) be tried for alleged corruption and killings by his armed militants.

At least four demonstrators and a foreign journalist were killed in the attack, and 30 people were injured.

"Three Marines fired from the palace in the direction from which the shots were being fired," Marine Maj. Richard Crusan told The Associated Press.