MANILA, Philippines (AP) — Three Filipino fishermen died in the disputed South China Sea after their boat was accidentally hit by a passing commercial vessel, the Philippine coast guard said Wednesday.

The fishermen’s boat, F/B Dearyn, was moored in a fishing area in the high seas off the northwestern Philippines when it was hit before dawn on Monday by the passing vessel and capsized, killing its skipper and two crew members. Three other fishermen survived, the coast guard said without elaborating.

The three fishermen died due to “possible head trauma” from the collision and their bodies were brought back to shore by the surviving fishermen, a separate police report said.

There was no indication that the deadly accident was related to the long-simmering territorial disputes in the busy waterway.

Philippine President Ferdinand Marcos Jr. expressed sadness over the deaths of the fishermen and said a coast guard investigation was underway.

“We assure the victims, their families, and everyone that we will exert every effort to hold accountable those who are responsible for this unfortunate maritime incident,” Marcos said in a statement.

An initial assessment based on the surviving fishermen’s account and sea traffic at the time of the accident indicated that a crude oil tanker under the flag of the Marshall Islands may have hit the Filipino fishing boat, the coast guard said in a statement.

The Philippine coast guard said it would try to locate the oil tanker and its crew as part of the investigation.