MANILA, Philippines (AP) — Philippine police disarmed villagers on a southern island and will test their weapons to determine whether they were used to massacre 15 fishermen at sea in a suspected feud over lucrative fishing grounds, officials said Wednesday.

The lone survivor, who was seriously wounded, told investigators that the gunmen ordered the victims to line up on their boats like a firing squad before opening fire.

Another fisherman who escaped the shooting but returned hours later described the grisly scene of bloodied bodies and bullet-riddled boats sinking. “They were merciless,” Jessie Boy Loquez said. “They fired at the heads.”

Regional police officer Felicisimo Khu said only four bodies have been recovered from the sea and the other 11 are missing and presumed dead. Loquez said the four recovered bodies did not sink or float away because the wounded survivor had managed to tie up their legs.

Basilan provincial military commander Col. Ricardo Visaya said the initial investigation indicated that the killing was triggered by a conflict over turf rights in the fishing ground.

Law enforcement in the area is weak, and deadly feuds fueled by business and political rivalry are rampant. Businesses often pay protection money to armed groups and hire private guards.

Residents of Sibago Island and their confiscated guns will be tested to determine if they fired the weapons in Monday’s attack on a group of rival fishermen, Khu said. He said no suspects have been named.

Visaya said the wounded survivor told investigators that the gunmen approached on three speedboats and ordered the fishermen to line up on their own boats before they were shot.

Visaya said that the gunmen resembled “a firing squad,” and that some of the victims jumped into the sea as they were fired upon.

Loquez and another fisherman escaped because they were in two small motorized outriggers away from the three main fishing vessels when the gunmen approached, Loquez told The Associated Press by phone from Zamboanga del Sur province north of Basilan.

Loquez said they had been fishing for tuna in the area for about 24 hours when his companion saw the gunmen, who were armed with M16 rifles. The pair fled; Loquez said they did not witness the shooting or hear the shots because of the noise from their engines.

He said they waited a few hours, until mid-afternoon Monday, before going back to the larger vessels, and saw two of them sunk after being peppered with bullets.

He said they brought the remains of the four fishermen to their hometown of San Pablo in Zamboanga del Sur province by midday Tuesday.

The 26-year-old, who got married less than a year ago, said he will stop fishing. “I will go back to planting sugar cane,” he said.