Charleston’s fire boat slammed into a harbor buoy late Saturday while racing to reach a stranded Navy patrol boat, officials said Monday, leaving the channel marker bent and mangled but still afloat.

The disclosure — confirmed by the Fire Department and the Coast Guard — marks the start of an investigation into what caused the impact that left a significant gash in the city’s $850,000 vessel.

The Navy patrol boat that originally triggered Saturday’s rescue has since sunk, slipping beneath the waterline near where it struck the Charleston Harbor’s south jetty.

The Navy also updated its injury report to say that six sailors who were on the 34-foot Dauntless were hurt, as opposed to the earlier count of three.

Three of the men were reported in stable condition. The others were treated and released.

Their names, rank and home towns were not released out of privacy concerns, the Navy said.

The events further illustrate the dangers of the harbor, especially after dark, where buoys, other boats, the jetties and other objects can cause serious accidents to fast-moving vessels.

In this case, both sides agree the city fire boat struck one of the harbor’s several channel markers — green buoy 25, which is positioned in the water between Fort Moultrie and Fort Sumter.

Photographs of the channel marker taken Monday show the buoy mangled and bent, with the light fixture at the top also broken into pieces.

Coast Guard Petty Officer 1st Class Lauren Jorgensen said Monday there were no reports before the accident that the light was out but that verifying its visibility, by sight and on-board electronics, will be part of the probe.

Under working conditions, the buoy emits a light blast every few seconds, she said, that can be seen across the water.

Charleston Fire Chief Karen Brack said that while the Coast Guard is taking the lead in the probe, the city will be contributing as well. “We want it to be a clean, clear process,” she said.

Five Charleston fire officials were on the boat at the time of the buoy collision. Brack declined to identify the captain but said he was experienced. The boat’s gash is less than 3 feet long, she said.

The incident began shortly after 10 p.m. Saturday when a Navy patrol vessel struck the very south tip of hundreds of feet of rock barrier that protects Charleston Harbor.

The sailors were members of the Coastal Riverine Squadron 10 from Jacksonville, Fla. They were headed to Charleston for what the Navy described as a long-range navigation exercise.

Few other details from the Navy side of the investigation were released, but spokeswoman Charity Hardison confirmed “boat captains are responsible for the safety of their craft and crew.”

She declined to discuss anything about the navigation equipment on board, citing operational security concerns.

Meanwhile, Charleston’s 36-foot fire boat, christened the “Louis Behrens,” remained out of the water Monday after multiple agencies responded and helped get the vessel back to port. City Council purchased the vessel last year, saying it was needed to respond to potential threats and fires on the water.

Saturday’s accident is one of several notable incidents that involved the Charleston Harbor approach. The deadliest in recent times was the loss of the sailboat Morning Dew.

The 35-foot sailboat crashed into the jetties in December 1997, killing a father, his two sons and their cousin.