Three of the four suspects in the killing of American marine biologist Kent Carpenter are now in police custody, the Philippine National Police (PNP) announced on Thursday.

PNP spokesperson Col. Allen Rae Co said during a press briefing at Camp Crame that those arrested were the alleged gunman, identified only as alias "M.O.," 26; alias "J.F.," who allegedly sexually assaulted Carpenter's live-in partner during the attack; and alias "C.D.," who allegedly served as the lookout.

"The Regional Director of Police Regional Office-Negros Island Region, Police Brigadier General Arnold Thomas Ibay Cardiño, reported to the Chief PNP that they have already arrested three of the four suspects in the killing of American marine biologist Kent Carpenter," Co said.

Carpenter, 73, was shot dead after armed men reportedly forced their way into his residence in Barangay Ajong, Sibulan, Negros Oriental, late Sunday night.

Authorities said Carpenter was watching television when one of the suspects allegedly shot him in the head before the group fled.

Carpenter was internationally known for his research on coral reefs and marine biodiversity. Authorities noted that his scientific work contributed to the Philippines' case in the landmark 2016 arbitral ruling on the West Philippine Sea.

Police are continuing operations to locate and arrest the remaining suspect.