NEW YORK (CBSNewYork) — There were warnings Friday about a possible upsurge in knockout attacks in which people are sucker punched for no good reason.

Video of one such incident shows a woman being suddenly kicked and then punched by a man. She was knocked out cold on the ground, but rather than help her several people took out phones to take pictures.

Nobody called the police.

It happened in Pittsburgh more than a month ago, and something similar happened in Brooklyn on Thursday night.

“As he went to tie up his dogs to confront these kids, the group distracted him from the front and then one came up behind him and hit him in the head,” a witness said.

The witness who asked not to be identified, said a group of teens leaving a Shake Shack in DUMBO singled out a 43-year-old man walking his dogs.

They tossed water and milkshakes at him before delivering a knockout punch to his jaw.

What happened next was even more disturbing.

“One boy ran up and crouched down and posed for a photo next to the guy that was passed out and then the rest of the group just kind of ran off and scattered,” the man said.

Witnesses said the victim lay unconscious for some 45 minutes, but in this case bystanders did step in to help.

Another similar incident happened last month in Manhattan.

Police are looking for three men wanted for the knock out style attack that happened on Essex Street on the Lower East Side. It left a 53-year-old woman with a swollen and bleeding face.

With no arrests yet in either case, people are worried.

“The police last night were telling me that this group of kids, it was the second time they had done it last night, and this seems to be this ongoing contest or trend amongst these teens,” one person said.