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The St. Louis region, already crippled by vast power outages Wednesday, came under a severe thunderstorm warning for the second time in three days.

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

Utility company AmerenUE said that fear was coming true for the city already in a state of emergency.

AmerenUE spokesman Tim Fox said the number of customers without power has risen to more than 400,000 as storms passed through the region a second time.

Friday morning, before the storm, 320,000 homes and business were still without power from Wednesday night's 80 mph winds and heavy rain. Progress with restoration was being made, and more than 200,000 customers had power back on within 36 hours.

The power company has said it could take four days before power is restored, but now it could take longer. Ameren Corp. has called the outage the worst in its 100-plus year history. As many as 500,000 customers lost power from the storm, which was followed by another day of near-triple-digit-heat.

More than 500 people spent Thursday night in two Red-Cross shelters, and a third shelter was scheduled to open Friday afternoon to take in people who could not stay in their hot homes, Rainford said.

"Saving lives and getting power back is our first priority," he said. "We can pick up the debris and assess the damage later."

Missouri National Guard troops arrived Thursday night and began looking for people needing help in hot homes without power, especially the elderly. They will spend the weekend or longer in St. Louis helping with debris removal, officials said.

The Guard, police, firefighters and volunteers were knocking on doors and offering bottled water as they checked on elderly residents again Friday morning.

Around the region, people began cleanup from Wednesday's storms. In St. Louis, the storms tore off a section of airport roof and dumped it on a highway. Windows were blown out of a hotel restaurant and a press box at baseball's Busch Stadium. At least three buildings collapsed, and more than 30 people were injured.

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"We're exploring the possibility of having guardsmen help Ameren in the actual power restoration efforts," Rainford said.

Gov. Matt Blunt sent the guard troops to St. Louis on Thursday and declared the city in a state of emergency. Blunt and other Missouri lawmakers have asked President Bush for financial disaster assistance.

On Friday, the weather was cooler — the high was expected to reach only the upper 80s and temperatures were expected to stay below 90 degrees through early next week.

Only two deaths have been reported so far in the region, including an East St. Louis, Ill., man died who die while walking near downed power lines and one heat-related death.

Enlarge By Seth Perlman, AP National Guardsmen gather in St. Louis Friday, July 21, 2006. Gov. Matt Blunt called in the Guard the same day the city was declared a disaster area after storms and power outages that affected 500,000 customers.