ORLEANS – Waltham City Councilor Pat O'Brien was enjoying a casual day on the beach when he saw something disturbing as he was getting out of the water.

"I turned to my right, and I saw all this blood shooting up into the sky. It was unbelievable," he said.

The sight caused O’Brien to spring to action, as he spotted two surfers near the growing collection of blood in the water, caused by an injured seal after a shark attack.

"I was in the water ... people started yelling ‘shark,’" said O’Brien, whose nephew recorded the aftermath. "I got out and saw blood coming out of the seal ... there was blood everywhere."

He quickly realized the surfers were in harm's way.

"I saw the seal kind of veer towards the [surfers], and I’m like, ‘This is bad. This is really, really bad,’" said O’Brien. "It kind of went into slow motion all of a sudden … And when I went in the water for a little bit, for a second I was thinking of my kids, my daughters. It was just so quick; it was all so fast."

O’Brien was visiting Orleans for the day, and said there were two surfers in the water when the shark appeared. One made it to shore while the other was helped by O’Brien and another man, he said.

Eventually, all swimmers were pulled out of the water at several beaches on the Outer Cape after sharks were spotted.

The seal attack that occurred near O’Brien happened at Nauset Beach, where O’Brien used to vacation in the 1980s and 1990s, but hadn't visited recently.

The aftermath of the attack on the seal, which was captured on video by bystanders, showed the seal swimming in a pool of blood close to shore as beachgoers and lifeguards yell for nearby surfers to get out of the water.

Watch video by Thayer Wade of the scene:

Shark attack at Nauset Beachpic.twitter.com/wR2fmCYjxE

— Patrick O'Brien (@PatrickOBrien36)August 21, 2017

The seal swam for a bit and eventually washed ashore, according to O'Brien.

The beach was closed to swimmers after the attack, according to an Orleans Police Department dispatcher. The Atlantic White Shark Conservancy's Sharktivity app showed a confirmed seal predation at 1:40 p.m. directly in front of the public beach at Nauset.

Ballston Beach in Truro was closed for one hour, or until about 12:30 p.m. after sharks were spotted there earlier, said Damion Clements, interim director of the Truro Recreation Department.

A notification was sent through the Sharktivity app at 11:31 a.m. warning of a white shark sighting close to shore at the beach. In 2012, a shark bit a Colorado man who was swimming off Ballston Beach.

Swimming was also prohibited at Longnook Beach between 12:30 p.m. and 1:30 p.m., Coast Guard Beach between 12:45 p.m. and 1:45 p.m. and Head of the Meadow Beach from 1 p.m. to 2 p.m. due to shark activity, according to the Truro Recreation Department's Facebook page.

On Sunday, Coast Guard Beach in Eastham was closed to swimming for an hour due to a shark sighting. According to the Sharkivity app, two great white sharks were spotted off the beach.

- Abby Patkin contributed to this report

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