While the gas piston is sent rearward, the bolt, attached to the gas piston, is unlocked from the chamber and ejects the spent casing.

Unlike a portcullis, which is raised and lowered vertically using mechanical means, yetts are hinged in the manner of a traditional gate or door, and secured by bolts attached to the yett, or by long bars drawn out from the wall or gateway.

Investigators believe a bolt attached to a metal tieback gave way on Monday night, causing the panels to fall and kill Milena Del Valle, 38, a restaurant worker from the Jamaica Plain neighborhood of Boston.

A rope with a bolt attached had been flung across and had caught in a fenestration of a projecting fragment of railing.

It used a bolt attached to a spring in a tube above the barrel.

Ripped upholstery showed bolts attached to the exposed metal walls, but the bolts had been cut or torn out.

In the blast-flattened grass, tie found a large piece of a support strut with the bolt still attached in one end.

She withdrew her head, removed from her handbag a ball of string with a heavy bolt attached to one end, laid it on the bed, picked up the binoculars and took up station by the window.