Atlanta ? Federal investigators were working through the weekend to start determining what caused a pedestrian bridge being built high above an Atlanta park to collapse, killing one man and leaving 11 other workers hospitalized, seven in intensive care.

Contractors were pouring concrete on the “canopy walk” at the Atlanta Botanical Garden when it crumbled Friday morning, sending workers hurtling up to 40 feet to the forest below. The U.S. Department of Labor’s Occupational Safety and Health Administration was investigating Saturday, but it could take months to determine what happened.

The 30-acre garden will stay closed through the weekend out of respect for the workers and their families, according to a statement on the Garden’s Web site. It is set to reopen Monday.