It happened Monday, Sept. 6 just before 2 p.m. in East Shoreham.
Jerry Comeau, age 28, of Selden, was riding a 2009 Harley Davidson eastbound on Route 25A, near George Avenue, with a group of three other motorcyclists, Suffolk County Police said.
One of the other operators collided with one of the other vehicles and fell from a 2005 Yamaha, causing a pileup, said police.
Comeau, who was at the back of the group, was ejected from the motorcycle, police said.
Comeau was transported to Stony Brook University Hospital in serious condition.
Another motorcyclist, Antonio Numa, age 29, of Centereach, was transported to John T. Mather Memorial Hospital in Port Jefferson with non-life-threatening injuries.
Neither Andrew Marz nor the fourth motorcyclist, William Marz, both age 28 of Selden, required medical attention.
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