A Great White Shark was filmed eating a seal near a packed beach as the sea turned red with blood.
It marked the first sighting of the summer season in New England, US, with officials warning beachgoers to be careful.
The incident comes two years after the region's last fatal shark attack.
The footage was captured by Will Little on a boat near Great Point Lighthouse in Nantucket, Massachusetts, on Sunday.
The shark can be seen thrashing around in the water as people behind the camera urge the seal to escape, shouting: "Go! Swim!"
But as the large predator strikes, one person says: "Oh god, you can see the blood!"
Another adds: "Holy f****!"
Blood can be seen filling the water, just a couple of hundred yards from the beach in the background of the video.
Shark season is just arriving, though is not expected to peak until late summer and early autumn in the region.
The clip was initially posted online by local news outlet Nantucket Current before being retweeted on Monday by the Atlantic White Shark Conservancy.
The AWSC tweet adds: "As the summer season approaches, remember to report your white shark sightings to the app Sharktivity!"
Crowdsourcing these "critical data points" helps to reduce encounters and promote safety, according to the organisation.
New England Aquarium is working with AWSC this summer to analyse video footage gathered by the app to help keep people aware of sightings.
Beachgoers and swimmers are being urged not to wade into outer waters over waist height, with white sharks spending much of their time in water under 15 feet deep, reports NBC Boston.
Purple flags warn of sharks in New England, with authorities keen to allow beachgoers to co-exist with them as the predators rarely ever actually attacking people.
The last deadly shark attack in the region saw Julie Dimperio Holowach, from New York, killed off the coast of Maine in July 2020.
Eyewitnesses said the 63-year-old appeared to be enjoying being in the sea with her daughter and was seen laughing before the attack near Bailey Island, Harpswell.
The Sharktivity app was developed alongside the Massachusetts Division of Marine Fisheries, the Cape Cod National Seashore, and officials from Cape Cod and South Shore towns.
It aims to raise awareness of the presence of white sharks off the coast of New England which consists of the US northeastern region comprising the states of Maine, Vermont, New Hampshire, Massachusetts, Connecticut and Rhode Island.
"The app sightings are fed by researchers, safety officials, and users that upload photos for confirmation," a statement on the AWSC website says.
"By enabling app users to report shark sightings and upload photos for confirmation, we are effectively crowd-sourcing critical data points on where sharks are spotted so as to reduce encounters and promote safety.
"Data from sharks with acoustic tags and Smart Position and Temperature Tags (SPOT) are also available on the app."
