It happened just after 3:30 p.m. Witnesses on the beach say Myers and his son were swimming and body surfing when they saw a distinct dorsal fin. A 911 call mentions a shark attack, but officials have not confirmed it. Shark experts were called the the scene.

"We could hear the youngest son apparently shouting, 'help, help, help us - help me,'" said Anne-Marie Corner, who witnessed the attack at Ballston Beach.

Myers' legs were bandaged up, and a squad of first responders carried him off the beach on a stretcher Monday afternoon.

"All of a sudden - between the two swimmers - we saw a fin come up and something came through the water. It was a very large fin, easily 15-inches high and came across and torqued a little towards the second swimmer, and within seconds - we realized it was a shark, and the swimmer had been attacked," Corner said.

A beach full of visitors - that witnesses say included a doctor and nurse - jumped into action.

"It was very apparent he was bleeding profusely from his feet and ankles," Corner said.

"It was like two people and then this large, dark blue, black thing came up and kind of torqued around, and you saw this big dorsal fin like went back down," said Walter Palmer.

Chris Myers - of Denver - was the man reportedly attacked by the shark. Ashton Abeyta - who has lived next door to Myers for five years - spoke to 9NEWS.

"He's always out on his bike," Abeyta said. "He's always sweating, playing with his kids, always out here playing football in the front. Just a super nice guy all around."

Abeyta was shocked when he heard about the reported shark attack.

"I was astonished," Abeyta said. "I was like, 'wow. Well ... he is from Colorado, so it is going to be weird to hear that he got attacked by a shark."

There have been concerns about sharks all summer in Massachusetts.

Earlier this month, a kayaker had a close call with a shark.

Since June there have been several shark sightings around seals near Chatham.

Officials have called in experts to try and figure out if Monday's attack was a shark, but witnesses at the beach say after what they saw, they're already convinced.

(KUSA-TV © 2012 Multimedia Holdings Corporation with NBC Universal)