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National Guard troops stepped up their search for people in hot homes without power to run air conditioning Friday as heavy rains and tree-toppling winds added to the misery of the worst power outage in the city's history.

"We have 55 percent of the residents without power. Our biggest fear is that the number will go up," said Jeff Rainford, spokesman for Mayor Francis Slay.

A heat wave that has baked much of the nation this week has been blamed for at least 28 deaths, three of them in Missouri.

The death toll in Oklahoma alone rose to seven. The state medical examiner's office said the heat caused the deaths of four elderly people on Thursday, including one in Oklahoma City, where the high that day was 107.

Oklahoma City was so hot that a portion of Interstate 44 buckled, forcing the temporary closure of two lanes.

In St. Louis, the weather has flip-flopped between sweltering heat and violent storms. As many as 500,000 Ameren Corp. customers in the area lost power Wednesday, making Thursday's heat that much more unbearable.

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Progress in restoring power had been made, but Ameren said the number of customers without power rose even higher Friday, to 570,000, as a new wave of storms passed through.

In northwest St. Louis County, winds from the latest storm tore the roof off an office building, causing concerns about a natural gas leak and leaving about 100 workers to fend for themselves in the rain.

Jeff Winkler, an analytical chemist at Severn Trent Labs, was just pulling into the parking lot when the roof came off.

"I saw the roof flying, and I was thinking, 'Please, don't hit my car,"' said Winkler, 26. "I thought I saw the worst of it earlier this week — but this was worse."

President Bush on Friday approved Missouri's request for an expedited disaster declaration, which mobilizes the Federal Emergency Management Agency and provides federal funding for debris removal and other emergency needs.

The power company had said Wednesday's outage was the worst in its 100-plus year history, and that it could take four days to restore power. On Friday it said the work could take even longer.

More than 500 people spent Thursday night in two Red Cross shelters, and a third shelter opened Friday afternoon to take in people who could not stay in their hot homes, Rainford said. Virtually every hotel room in the region was booked for the weekend, mostly by residents taking refuge from homes without power.

High temperatures in St. Louis had dropped to the mid-80s Friday, but National Guard troops, police, firefighters and volunteers were knocking on doors that morning to check on elderly residents and offer bottled water, and more than 50 cooling centers were set up in the area. On Thursday authorities said a 93-year-old St. Louis woman had been found dead in a home without power to run the air conditioning.

Tens of thousands of people also were without power in parts of southern Illinois that were pounded by storms for the second time in three days Friday.

The National Weather Service said winds estimated at up to 65 mph cut through Jefferson County, and that it had received reports of a tractor-trailer toppling and modular homes from a dealership being blown onto Interstate 57.

A Jefferson County sheriff's dispatcher said officials were too busy to discuss damage reports.

Conoco-Phillips' biggest refinery, northeast of St. Louis in Wood River, Ill., will remain idled into next week because of damage from Wednesday's storm, officials with the Houston-based company said. It was unclear whether gas prices would be affected by the shutdown of the refinery, which can process 306,000 barrels of crude oil per day.

Weather in Missouri and Oklahoma was expected to be relatively cool over the weekend, a relief after days in which several people died in sweltering conditions. Heat-related deaths also have been reported this week in Illinois, Kansas, Pennsylvania, Arkansas, Indiana, South Dakota and Tennessee.

In New York, tens of thousands of people were still without power Friday, the fifth day of a mysterious electrical problem during the hottest week of the year.

Consolidated Edison spokesman Chris Olert said the power company was making every effort to get the situation fixed but couldn't estimate when that might happen. He said the company didn't know why things went wrong.