A slab of wet concrete collapsed on the 24th floor of a high-rise construction site Friday, injuring 17 people, seven of them seriously, authorities said.

Rescuers had to shore up the fallen concrete to free two trapped construction workers. The last one was pulled out two hours after the collapse.

The injured had to be carried down several flights of stairs on stretchers because the elevator did not go all the way to the 24th floor, said Capt. George Williams of Arlington County's technical rescue team. Surgeons were brought in to treat victims.

The concrete came down from the roof of the 300-foot office tower after the scaffolding holding it up gave way, said Tom Polera, a spokesman for the Arlington County Fire Department.

The cause of the accident was under investigation. An initial review showed no major safety violations at the work site, said Jennifer Wester of the state Department of Labor and Industry.

The accident happened the day the contractor, Clark Construction, had planned to hold a "topping-off party" to celebrate the completion of the pouring of concrete, said Brian Apt, a company official.

Oscar Moscoso, a worker at the site across the Potomac River from Washington, said he pulled out six people with his hands and shovels.

Three workers, including the two who had been trapped, were in critical condition and four were in serious condition, authorities said. Nine other construction workers and one firefighter suffered minor injuries.

Roughly 200 workers were at the site when the concrete gave way.