The garage is part of a $245 million expansion project at the Tropicana Casino and Resort.
According to New Jersey Gov. James McGreevey, search and rescue teams continue to hunt for missing workers. He also said the remaining garage is not a stable structure and there is the potential for a second collapse.
Robert Levy, director of emergency management for the city, said officials were sending search cameras to look through the rubble and called the accident "one of the worst collapses Atlantic City has ever seen." The Occupational Safety and Health Administration was leading the investigation.
Bill Crilley, 42, an insulator working at a project in another part of the city, said he rushed to the building after hearing the collapse and saw authorities carrying one body away.
"It's ugly. Horrific. The whole stairwell is crushed," he said.
The 2,400-space, 10-story parking garage flanks an 18-story hotel tower, also still under construction. The end of the building opposite the tower collapsed at about 10:40 a.m., leaving five layers of concrete and steel sloping downward at a steep angle.
"I heard all those floors go. The whole tower shook, like it was a miniature earthquake," said Jim McNeill, 56, a caulker who was working on the building when it collapsed at about 10:40 a.m.
McNeill said he looked down from the tower and saw five men lying on the ground. Only one was moving.
Sixteen people were at Atlantic City Medical Center, including 10 in the trauma unit. Four people were taken to Shore Memorial Hospital in Somers Point.
Maureen Siman, a spokeswoman for the casino, called the accident "a tragedy. We're devastated. We just hope the workers are OK."
Workers gathered on the street nearby to gaze up at the collapsed frame. In the crowd, a distraught woman searched for her husband. "He was working up there last night, I know," she said.
The Tropicana expansion project was scheduled for completion next March. Named "The Quarter," the project is intended to diversify the casino's offerings with forms of entertainment other than gambling. Tenants slated to move in include a magic shop, an IMAX theater, a Latin-themed nightclub and eight restaurants.