St. Louis ? 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 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.

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

Members of the Missouri National Guard remove a fallen limb off Kingshighway as they help check the welfare of residents in St. Louis, Mo., neighborhoods. Gov. Matt Blunt mobilized the troops to help the St. Louis area recover from Wednesday night's widespread storm damage.

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.