SAN DIEGO -- About 25 people were standing in the walkway to escape the hot, August sun when its wooden walls and roof came crashing down. Some scaffolding along the 3-story building also fell.
"The walls started moving, then the bang. Everything started coming down. Everyone started screaming," said Abigail Reckermann, 50, who went to the hospital with a swollen ankle.
Ariel Medina, 34, was uninjured but saw a board fall onto the back of a man she was talking with. "It was a living nightmare," Medina said. "The whole thing just caved in. People were trapped."
Three people suffered life-threatening injuries, including head trauma, when they were hit by falling debris, authorities said.
Father Joe Carroll, president of St. Vincent de Paul Village, said many of the injured had just eaten lunch or were living at the homeless shelter he runs across the street. The shelter had served lunch to about 1,000 people.
Carroll said his staff recognized some of the shelter clients as they sat on a nearby curb with neck braces. Others were bleeding lightly as they waited for an ambulance.
"They either had a meal here or live here and were walking to the trolley," he said.
Fire spokesman Daniel Calderon said investigators did not know what caused the collapse.
The walkway was located along a three-story apartment building under construction in downtown San Diego. Shopping carts full of plastic recyclable bottles could be seen in the rubble.
Morris Choo, 35, heard the crash from a store where he was buying a lottery ticket moments after chatting with fellow shelter residents along the walkway. Choo ran outside to find a trapped friend who recently moved from Chicago with his wife and was trying to get back on his feet while battling epilepsy.
"His whole head was split open," Choo said.
Julie Hattler, chief financial officer for Affirmed Housing Group, the building's developer, said her company hadn't determined the cause of the collapse.
"There's not much to say other than we have to find out what happened," she said.
Affirmed Housing Group identified Allgire General Contractors in Carlsbad as the builder of the apartment complex. A woman who took a call at Allgire's offices said she could not comment then hung up the phone.
Allgire General was fined three times over the last decade for safety violations, according to U.S. Department of Labor records.
The largest fine was a $15,750 penalty issued in December 2004 for not having guardrails on an elevated platform or stairway at a work site, California Division of Occupational Safety and Health spokesman Dean Fryer said.
Allgire was also fined $5,000 in February 2005 for not reporting an accident, and $150 in April 2002 for using an electrical outlet box without a cover, Fryer said.Associated Press writer Jacob Adelman contributed to this report.