Four crew members died of suspected asphyxiation after methane gas reportedly built up inside a vessel docked at a local shipyard, officials said.

The victims were working inside a storage area of a vessel at the Mega Fishing shipyard sardine processing plant in Barangay Cawit at around 7 a.m. when they experienced dizziness and fainted.

Emergency responders rushed the workers to a nearby hospital, where physicians declared them dead on arrival.

Three of the victims were identified as Ranillo Sanugal Jr.; John Edgar Dotillos and Arnold Bongcawel. Authorities are still working to identify the fourth victim.

"They just went to work normally this morning. We are grieving deeply over how fast everything happened," a relative of one of the victims said.

Investigators are looking into a lack of oxygen or a toxic buildup of methane gas within the confined space as the likely cause of death.

Zamboanga City Mayor Khymer Adan T. Olaso inspected the shipyard Thursday noon and ordered a full-scale joint investigation into the incident.

"We entered normally to work this morning. Our grief is immense given how fast things happened," a relative of one of the victims said.

Olaso directed City Health Officer Dr. Dulce Miravite to form a joint investigative team to determine if the company violated occupational safety and health standards.

Meantime, officials from the Philippine Coast Guard, local police, forensic investigators, sanitary inspectors and village leaders joined the mayor during the on-site assessment of the vessel's containment area to check for safety lapses.