Rescue workers sawed away at debris under the glare of flood lights, trying to find an estimated five to eight people who were in the Caribbean Market when it collapsed, at one point asking for silence so they could listen for victims.
The search got underway as bodies retrieved from the site before the collapse -- those killed in the quake last month -- lay nearby covered in white sheets and awful smells hung in the air.
"There were looters inside the building," site supervisor Meir Vaknin told AFP. "I was trying to get rid of them and when the building fell there were some of them inside."
He estimated five to eight people had been inside and said at least one was spotted alive after the collapse.
A Mexican rescue worker, Carlos Mendez, said they had found two people so far. Asked if they were alive, he said, "This is what we are going to find out."
The five-story building had been popular with well-off Haitians and was the capital's largest supermarket.
It was badly damaged in the devastating January 12 earthquake that killed more than 200,000 people, but remained partly standing.
Vaknin said the collapse occurred Tuesday as he was working at the site with an excavator to remove bodies still there from the quake. A large excavator could be seen inside a hole in the ground.
"I was sitting in the excavator when it fell in," he said. "I'm so lucky to be alive."
He said no one from his crew was hurt.
At least two dozen rescue workers were at the scene, with US military and UN police sealing off the area.
Rescue workers were using cutting tools to try to reach people. Sparks flew as they cut with a saw then pulled away a tangled ball of metal.
The collapse came after the already stumbling relief effort was dealt a potential new blow earlier in the day when the WHO stopped providing free drugs to private clinics and NGOs after reports patients were being charged.
This followed distressing scenes of hungry survivors rubbing their bellies and shouting desperately on Monday after the UN suspended food supplies to some 10,000 quake survivors in the capital when fake coupons were discovered.
A WHO spokeswoman said there would be a review after three months and stressed that the new rules would allow the UN's health agency to keep a closer watch over its drug stocks.
Meanwhile, doctors treating a frail Haitian man said they believed he survived 27 days buried in rubble after the quake, but there was no explanation of how.
The rescued man, named as Evans Monsigrace, told doctors at a University of Miami field hospital in the capital that he had been buried by the quake while cooking rice.
"Amazingly he got out after 27 days. It's amazing and we are proud to have him here," said doctor Dushyantha Jayaweera, the chief medical officer at the center.
It was not immediately possible to verify Monsigrace's claim and there was no explanation for how he survived so long if he was trapped under the rubble without water.
The emaciated survivor was brought into the hospital on Monday and treated for dehydration by emergency doctors, Jayaweera said.
According to the man's mother, he was discovered by people clearing debris who then alerted Monsigrace's brothers.
Some 135 people are known to have been saved from the rubble by international rescue teams since the quake.
The last person rescued in Haiti was a 16-year-old girl pulled out almost two weeks ago.
Angelina Jolie injected star power into the relief work, touring the refuge caring for the children at the center of an American kidnap case.
The center is caring for 33 children who 10 Americans have been charged with kidnapping after they attempted to take them across the Dominican border in a bus.
Haitians whose children wound up with the Americans told the judge in the case Tuesday they gave the missionaries permission through a Haitian pastor to take them, the Americans' lawyer said.
One man, who did not want to give his name, said before entering the hearing that he had handed his 15-year-old son over because the boy had "fractured his foot in the earthquake" and needed treatment.
The Americans were arrested on the border with the Dominican Republic on January 29 as they traveled with the busload of children and were charged last week with kidnapping and conspiracy.
They have claimed they had no ill-intent in taking children they thought were orphans.
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