BOSTON (WHDH) -- A shark biologist said Tuesday it likely was a great white shark that bit a swimmer off a Truro beach Monday.

State officials said Tuesday he hasn't talked to the victim, but said the description of his injuries, eyewitness accounts, and the presence of gray seals near the beach fit with a great white attack.

The attack happened at Ballston Beach, which remained open Tuesday as authorities continued investigating. Town officials put up signs urging swimmers to take caution while in the water.

Witnesses say they saw the man swimming with his son when they saw a distinct dorsal fin and then saw something rise up out of the water and strike the man, who then started swimming frantically back to shore.

One man raced into the water on a boogie board to help the victim and a vacationing nurse ran to help bandage the victim’s legs.

“I was standing there just about to sit down and his legs were completely covered in blood, and you could see the flesh wounds from like 20 yards where I was from it,” said Seth Blaustein.

“Once they got back to the beach, that’s when they realized that someone was laying down and there was just blood all over his legs. So immediately there was another nurse there first. She grabbed the boogie board, elevated his legs, and once I saw someone was laying down and it wasn’t just that they had difficulty getting back to shore, I ran over and I helped with the towels and holding pressure to the wounds to control the bleeding,” said Cassie O’Connor.

Wednesday morning, state officials said the bites were confirmed to be consistent with a Great White shark attack.

Greg Skomal of Massachusetts Wildlife said, "It's the size of the injury that's really important here. Both the individuals' calves were struck at the same time."

Marine biologists said shark sightings could become more frequent along the Cape because of the growing gray seal population. The last time some one in Massachusetts was attacked by a great white shark was 1936.

The victim is being treated at Mass General Hospital. He's recovering, but the hospital is not releasing any information on exactly how he is doing.