The agency tweeted that purple flags are up at the Nahant Beach Reservation to warn swimmers about the danger as the lifeguard service has ended for the season. The MCR did not give any description of the shark or what species it could be.
It's been a busy shark season in Massachusetts this year.
Beachgoers at Plum Island spotted a great white shark in July. A hammerhead shark put on a show for boaters in Nantucket when it attacked a fish near the water's surface in August.
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