Steve Libert told The Associated Press that his crew found the debris this month about 120 feet from the spot where they removed a wooden slab a year ago that was protruding from the lake bottom. Libert believes that timber was the bowsprit of Rene-Robert Cavelier, Sieur de La Salle's ship, although scientists who joined the 2013 expedition say the slab more likely was an abandoned fishing net stake.
VIDEO: Hundred-year-old shipwreck found off San Francisco coast
He acknowledged his dive team had found no "smoking gun" such as a cannon or other artifacts with markings identifying them as belonging to the Griffin. But the nails and other implements appeared similar to those from La Belle, another of La Salle's ships that sank near the Gulf of Mexico, Libert said.
He said his organization has sent images of the debris to three French underwater archaeologists who took part in last year's search, and that he hopes state and federal permits can be obtained to excavate in the area in September.
The French team was led by Michel L'Hour, director of the Department of Underwater Archaeological Research in the French Ministry of Culture and an authority on shipwrecks. L'Hour told the AP by email Tuesday that the latest findings were "encouraging" but that more evidence was needed to determine the origin of the items.
"The wooden remains that have been observed could correspond to a wreck," L'Hour said.
They include treenails with wedges and square nails that have some similarity with La Belle's fasteners "and a few other details already observed on wrecks dated in the 17th century," he said.
But he said the artifacts that have been seen could be dated as late as the 19th century and that items such as ceramic shards are needed to provide more certainty.
"We are always interested in participating to assess the site," L'Hour said, adding that the U.S. and France would need to approve any new involvement in the project by his team, which comprises civil officers of the French government.
Dean Anderson, Michigan's state archaeologist, said Monday he hadn't been notified of the find and could not speculate about whether the Griffin had finally been located. Anderson supports the theory that the timber discovered earlier was a fishing apparatus.
The area strewn with debris is roughly the size of a football field, said Brian Abbott of Nautilus Marine Group, who joined Libert's search this month and took sonar readings of the bottomlands. It is near tiny Poverty Island in northwestern Lake Michigan and about 50 feet below the water's surface.
The Griffin is believed to be the first ship of European design to sail the upper Great Lakes. It disappeared with a crew of six on its maiden voyage in 1679 after La Salle had disembarked near the mouth of Wisconsin's Green Bay