Clogged roof drains may have contributed to the collapse of a department store during a torrential rain, officials said Saturday, while rescuers dug cautiously through the rubble with little hope of locating more survivors.

By Saturday morning, workers had reached the first level of the three-story building and had cleared more than half the debris from Thursday's collapse.Two adults and four children were rescued from the massive pile of rubble Friday, but 16 people died and at least 47 were injured. Three of the victims have been found but not removed from the debris of the Amigo Store.

Clogged drainage holes on the building's flat roof could have contributed to the collapse by allowing rain water to accumulate and possibly overload the building, said Kermit Cronack, tax assessor-collector for Cameron County.

"If you were the owner of the store, how many times would you go up and see if the drainage pipes were clogged, especially if it's a three-story building?" Cronack asked.

Tony Yzaguirre Jr., a city tax assessor-collector, said the building was inspected earlier this year by the fire department, as it is annually.

Minimal construction standards contributed to the tragedy, other officials charged.

"That old store was nothing more than a stack of blocks," Cromack said. "This was bound to happen sooner or later."

The third floor of the 20-year-old building was added in 1975 and Cronack said the building's construction was average for one that age in the area, explaining "we don't build as heavy as they do elsewhere because we don't have snow."

"The roof is not designed to carry all that weight," Brownsville building inspector Fernando Romero told The Dallas Morning News. "It's a tragedy and one that may happen again at any time."

Mayor Ygnacio Garza said investigations were being launched by the city, the building's insurance company and the federal Occupational Safety and Health Administration.

Garza said the rainstorm, which poured 2 inches in less than 30 minutes, may have been a factor, but that other factors also may have played a role.

"We have a report from witnesses that a bolt of lightning struck behind the building and hit some transformers and perhaps knocked them down," Garza said.

Rescue workers concentrated their efforts Saturday on two sites in the rear of the building, where U.S. Border Patrol drug- and people-sniffing dogs indicated spots where people, living or dead, might be trapped. Workers formed human chains to remove debris basket by basket.

Juan Garcia, assistant chief U.S. Border Patrol agent in McAllen, said the dogs located nine bodies Friday. "The dogs are so well trained they know for sure there's a body down there," said Garcia.

"Right now we don't have much hope we'll find anybody alive, but there's always a chance, especially in the back where there is corrugated metal that could have given them some air," said Monty Jenkins, coordinator of the Special Medical Response team from the U.S. Bureau of Mines from Pittsburgh.

Rescuers were removing chunks of concrete, broken cinderblocks, shattered glass, merchandise and splintered tables.