11:00 AM PT -- Nassau County PD Commissioner Patrick Ryder tells TMZ ... "Reports of explosives being found at the site are unfounded. There is a person who is being questioned who may have been training a bomb detection dog near the site. The individual with the bomb dog falsely reported explosives being found and that individual is currently being detained by the police."

After two assassination attempts on Donald Trump's life this year, a fresh threat was reported Wednesday -- and though it's been dismissed as bogus, it’s probably not doing much to ease  Trump and his team’s nerves.

This morning, journalist James Lalino claimed in an X post ... Nassau County Police Department sources told him the perimeter at Trump’s Long Island rally site at Nassau Veterans Memorial Coliseum was apparently "breached" and a "blue barrel" was removed.

Lalino also claimed during a K9 sweep, authorities supposedly found an explosive device in one of the vehicles nearby, and the driver fled into the woods.

NBC News Correspondent Tom Winter quickly debunked the claim ... saying on X the report was "erroneous" and that multiple law enforcement officials told him and his team there was no evidence of any explosive device being found. A Nassau County Police spokesperson later confirmed no explosives were found.

It’s still a big security scare for Trump, especially with thousands of MAGA fans about to pack the venue to hear him speak tonight.

This comes just days after suspected would-be assassin Ryan Wesley Routh was arrested for allegedly lying in wait with an AK-style rifle in bushes only a few hundred yards away from where Trump was golfing in West Palm Beach, Florida.

That was the second attempt on Trump's life this year. On July 13, 20-year-old Thomas Matthew Crooks was shot and killed by a Secret Service sniper after he opened fire at Trump's campaign rally in Butler, Pennsylvania.