Moore, Okla. ? The morning after a tornado destroyed more than 300 houses and businesses in and around Oklahoma City, firefighters went from one wrecked home to another Friday and spray-painted a big, red X on each roof if no one was trapped.

There were a lot of X’s: Grateful authorities said that remarkably, the twister that carved a 19-mile path through the most densely populated part of the state did not kill a single person.

“We were lucky, basically,” said Gary Bird, deputy fire chief in this southern suburb. “We had warning from the National Weather Service, and Moore had replaced and upgraded all of its warning sirens, so that helped a lot. People pay a lot more attention.”

But late Friday, another tornado touched down in the Oklahoma City area. Storm spotters reported the tornado touched down along Interstate 40, damaging a Xerox plant. The area was under a tornado watch until early today.

The state insurance commissioner estimated damage at more than $100 million. But in a week where tornadoes killed 42 people in Missouri, Tennessee, Kansas and Illinois, officials here were counting their blessings.

More than 100 people were treated for injuries after the tornado, but only 21 remained hospitalized Friday, said Paul O’Leary, a spokesman for Oklahoma City’s ambulance service. At least three were in critical condition.

The tornado ripped through southern Oklahoma City with winds the National Weather Service measured at 200 mph.

The General Motors assembly plant, background, in Oklahoma City is heavily damaged by a tornado that ripped through the metro area. There were no reports of serious injuries at the plant, thanks to an early warning received Thursday by management.

Blaring tornado sirens and TV and radio bulletins provided 10 to 20 minutes’ warning in most cases, enough time for thousands of people to scramble into shelters before the twister peeled off roofs, blew away walls and uprooted trees Thursday afternoon.

It was the third twister to hit Moore in six years.

“I just can’t imagine why it keeps coming down the same path,” said Shearon Cunningham, standing outside her daughter’s home, which had caved-in walls and no roof. “Every day in May and June you live on pins and needles.”

About 50 homes and 10 businesses were heavily damaged or destroyed in Oklahoma City. In nearby Choctaw, nearly 100 homes were damaged and 10 to 15 were destroyed.

The heaviest damage was in Moore, where the tornado chewed up a path a quarter-mile wide and 3 1/2 miles long. Bird said 300 homes and 35 businesses were destroyed.