PORT GAMBLE — A 91-foot-long tugboat has been identified as the source of a diesel spill that took place in Gamble Bay sometime between Sunday and Monday morning.

The “Agate” was moored at one of the piers located within the old Port Gamble mill site and had a 500-gallon tank on board, said U.S. Coast Guard spokesman Adam Eggers. Leaking had stopped by Monday afternoon but responding officials were uncertain as to whether any fuel remained in the tank. The agency estimates about 100 gallons of red marine fuel had leaked into the water. It is unclear exactly when the vessel sank.

Coast Guard divers were expected to start investigating the bay at first light Tuesday morning, Eggers said.

As the Coast Guard and Washington State Department of Ecology investigated Monday, there was another sunken vessel found nearby, however, it had no oil in it and it was unclear as to when it had gone under, Eggers said.

The Agate’s owner has been identified, however, because the incident is under investigation, his or her name is not being released, Eggers said. The Coast Guard is now trying to determine the cause of the spill, finding out why the boat sank and whether it was by accident or negligence.

The Coast Guard received a call about the spill around 9:30 a.m. Monday but crews didn’t depart from their Seattle office until 10:30 a.m. because officials were conference-calling with a source who was on the scene, providing more information that later helped investigators when they arrived in Port Gamble.

The Coast Guard’s contracted spill clean up crew, NRC Environmental Services, arrived on the scene around 3 p.m. Employees of Caicos Construction, which leases waterfront property on the mill site, had already set up its booms to contain the oil before NRC arrived. It had been reported that the sheen of the oil had reached as far north as the Driftwood Key neighborhood.

While the Coast Guard started the initial clean up process, it will eventually turn the responsibility over to the appropriate party once it is determined.

While it is still unclear about who is responsible, Port Gamble S’Klallam Tribal Chairman Ron Charles said the tribe has its own concerns as the tribe’s reservation and one of its primary fishing areas is located directly across the bay from the mill site. Charles received word of the spill at daybreak Monday, after tribal members reported a strong smell of diesel in the bay and saw red material being washed upon the shore.

A light sheen could be seen on the water as the tide came in midafternoon, and clumps of red material had built up within the creeks that feed into the bay.

Besides Port Gamble, the Skokomish, Jamestown S’Klallam and Lower Elwha S’Klallam tribes harvest shellfish and salmon from the area.

“Between the salmon and the shellfish, it’s a multimillion dollar industry,” Charles said. “It has the opportunity to be devastating.”

The tribe’s fishing nets were removed earlier in the day once the tribe received news of the spill, he added.

Port Gamble S’Klallam hydrologist Dave Fuller said the tribe will have to wait for tests to be completed of the water before it is determined safe to harvest from the area again.

The spill seems to be much larger compared to the oil that spilled from a partially sunk boat that was found in the bay last December and had leaked about 50 gallons of diesel fuel, Charles said.