Philippine National Police chief Gen. Jose Melencio Nartatez Jr. on Thursday commended police units for the swift arrest of three suspects in the killing of American marine biologist Kent Carpenter and ordered an intensified manhunt for the lone remaining suspect.

Nartatez praised the operating units whose investigation led to the arrest or surrender of the suspects identified only by their aliases "M.O.," the alleged gunman; "J.F.," who allegedly sexually assaulted Carpenter's live-in partner; and "C.D.," who allegedly served as a lookout.

The suspects were apprehended by operatives of the Police Regional Office-Negros Island Region, the Negros Oriental Police Provincial Office, the Criminal Investigation and Detection Group, and other police units days after Nartatez ordered a thorough investigation into the case. The directive was issued in line with President Ferdinand Marcos Jr.'s order, through Interior Secretary Jonvic Remulla, to intensify the campaign against violent crimes.

Police are still pursuing a fourth suspect known only by the alias "Eden," who allegedly took the group's loot following what authorities believe was a robbery that turned deadly.

"The swift arrest of three suspects reflects the dedication and professionalism of our investigators. Our work is far from over. Our priority is to complete the case by arresting the remaining suspect, ensuring all evidence is properly documented, and building a strong case that will secure the conviction of everyone responsible," Nartatez said.

Investigators said the suspects allegedly entered Carpenter's residence in Sibulan, Negros Oriental, on 12 July. Initial findings indicate the crime was a robbery that escalated into homicide.

Authorities said one of the suspects had previously worked at the victims' residence and allegedly used that opportunity to familiarize himself with the property before the attack.

Police are also determining whether the suspects were involved in previous robbery incidents in the area.

At this stage, investigators said they have found no indication that the killing was connected to Carpenter's work as a marine biologist or his reported contribution to the Philippines' victory in the 2016 South China Sea arbitral case.

"I have directed all concerned units to sustain intensified manhunt operations until the remaining suspect is arrested," Nartatez said.

"We are maximizing intelligence gathering, coordinating closely with neighboring police units, and encouraging the public to provide any information that can lead to his immediate capture. We will not stop until every person involved in this crime is brought before the courts," he added.

The three arrested suspects are facing robbery with homicide charges, while authorities continue forensic examinations related to the alleged sexual assault on Carpenter's live-in partner.

The PNP appealed to the public to report any information that could lead to the immediate arrest of the remaining suspect.