BOSTON — A cyclist in a black hooded sweatshirt rode down a Sumner Street sidewalk in East Boston around 3:30 p.m. Friday, sailed past an elderly man leaning on his walker, and clipped a sidewalk table with his handlebar as he went. Then he did something that turned a bad sidewalk moment into a police case: he stopped, wheeled his bike around, ran back the ten or fifteen feet of pavement he had just covered, and slammed the old man off his feet and onto the concrete.

Then he got back on the bike and disappeared into the East Boston street grid.

The elderly man took himself to a hospital. His injuries were non-life-threatening. His dignity is another question.

The cyclist could have kept going — he had already passed the old man, he was already down the block, he was on his way somewhere. Instead he wheeled around, dismounted, and closed the distance on foot back to a man who could barely stand on his own. Then he drove him to the pavement, on camera, in broad daylight, in front of a parked car and a Sumner Street storefront.

Then he got back on the bike and pedaled off like nothing had happened.

That's it. That's the description.

The surveillance camera got him from one angle — the storefront label reads "Outside Facing Right." Boston Police are investigating. No arrest as of this writing.

Within hours of the video posting, East Boston community pages had the block, the corner, and the storefront pinpointed. Neighbors were passing the clip around, tagging each other, asking whether anybody recognized the guy in the hood.

Somebody does.

If you're one of them: call Boston Police District A-7 at 617-343-4220. Anonymous tip: CrimeStoppers, 1-800-494-TIPS.

The elderly man got up. The cyclist rode off. Neither of those is over.