Chicago ? Police released video Friday from the nightclub stampede that killed 21 people, a horrifying frame-by-frame account showing panicked crowd members screaming and crushing each other in a desperate attempt to get out.

The halting, grainy footage — captured on surveillance cameras at the E2 club and shown to the media for the first time — shows police and firefighters grabbing arms in a vain attempt to pull victims from a mass of interlocked bodies that jammed the stairwell a few feet from the club’s open front door.

“All efforts to try and pull people out were unsuccessful because of the weight of the crowd,” said Chief of Detectives Philip Cline. “There’s actually people stacked up. It was about four people high.”

Authorities estimated several hundred people pushed into the stairwell early Feb. 17 after a guard used pepper spray to break up a fight on the dance floor. All the victims died there.