PORTSMOUTH — Fire officials have concluded their investigation into a July blaze that destroyed a pier at the Mount Hope Marine Terminal, but still haven’t determined what caused it.

They did, however, rule out arson.

“With the information we have available to us right now, we believe it was an accidental cause,” said Chief Jeffrey Lynch. “The point of origin appears to have been below the pier.”

Lynch said officials found no evidence of accelerants and no indication that electricity or fuel started the fire. Witnesses’ statements failed to pinpoint a cause, he said.

“We’ve exhausted all of our investigatory options,” Lynch said.

Deputy Chief Philip Simmons, who took part in the investigation, echoed the chief, saying, “The cause was undetermined … because of the lack of available forensic evidence and the amount of damage to the pier.”

Because no cause was found, the case remains open, he said.

As for talk that smoking materials started the fire, Simmons said, “That’s a rumor, and it was unsubstantiated.… We can’t go on supposition.”

Simmons said the fire caused $6 million worth of damage to the pier. The fire also destroyed a 65-foot fishing boat owned by Richard Mudd, of Tiverton, which was tied to the pier, and a crane, owned by Sheltow, a marine contractor based at the terminal.

“We believe the crane had nothing to do with the origin of the fire,” Lynch said.

The fire erupted on the evening of July 3, producing flames that could be seen across Narragansett Bay and bringing 100 firefighters to the scene from Warren, Swansea and Jamestown.

The pier, about a mile south of the Mount Hope Bridge on the west side of Aquidneck Island, was once a deep-water port where the Weyerhaeuser Corp. unloaded Douglas fir shipped from the West Coast.

O’Neill Properties Group purchased the 44-acre Weyerhaeuser property and plans to build a total of 152 units of luxury housing that would be an extension of the Carnegie Abbey Club immediately to the south. The old pier, which Simmons said was once the largest all-wood structure of its kind on the East Coast, was to serve as the core of a new marina at the development.

rsalit@projo.com