Philippine police announced Thursday the arrest of three suspects in the killing of a prominent American marine biologist in his home in central Philippines.

Three men broke into the home of Kent Carpenter, 73, in the coastal town of Sibulan, Negros Oriental province, in what is suspected to have been a robbery on Sunday.

Police officials said it was reported that one of them shot the biologist in the head with a pistol while he was sitting on a couch. They added that another suspect forced Carpenter's Filipino girlfriend into a room and raped her.

National police spokesperson Colonel Allen Rae Co said the suspects took a laptop computer, an undisclosed amount of cash, and a backpack before fleeing.

Co added in a press conference that investigators are trying to determine why the suspects shot Carpenter.

A fourth suspect, who acted as a lookout outside the house, surrendered to police and is said to have provided important details about the crime.