CHICAGO, AUG. 28 -- A cluster of tornadoes swept a 12-mile path from Oswego to Joliet this afternoon, killing at least 20 people and injuring more than 280 as the storm demolished subdivisions, collapsed a high school into a brick pile and tossed people from their houses into cornfields.

At least seven people were reported killed in Plainfield, one just north of Plainfield and two in unincorporated Joliet near the Louis Joliet Mall. At least 10 people died in Crest Hill, where walls and roofs were peeled from at least three apartment complexes, according to Will County deputy coroner William Fergusen.

Tonight, emergency workers were picking through the rubble left by the deadliest twisters to hit northern Illinois since 1967. Emergency workers said they expect the death toll to rise as searchers make their way through the rubble and because some of those injured were in very critical condition.

The cold front that spawned the tornadoes is expected to arrive in the Washington area late Wednesday, but National Weather Service forecaster Ed Schoenberg said tonight that it was too early to say if the front might trigger severe storms in the area. "It's possible, given the history," he said. ". . . We're certainly going to keep watching it."

He said the forecast is for a 50 percent chance of showers and thunderstorms.

A pilot who flew over the area shortly after the storm said he saw a single line of damage that stretched for 12 miles from Oswego, through Plainfield, across Interstate Highway 55 and into Joliet.

The tornadoes spun rapidly out of a large, violent thunderstorm that took just minutes to make its way along a southeasterly path.

The twisters left behind a swath of wreckage ranging in width from 200 yards to one mile, and they turned the area inside out, tossing branches into houses and throwing kitchen tables onto roads.

Overturned cars and uprooted trees were scattered along many roads, including Interstate 55 and U.S. 30.

"I saw people actually blown 100 to 200 yards into a cornfield nearby," said David Brozman, 38, a paramedic for the Joliet Fire Department who was dispatched to the intersection of I-55 and U.S. 30.

"They were just lying there. The tornado had blown them out of their homes. And on the ground next to them were sticks and bricks."

Late tonight, Gov. James R. Thompson (R) told Will County officials that Illinois National Guard troops would be sent to the area to prevent looting. By 10 p.m., police had arrested five people for looting, according to Don Gould, Will County emergency services director.

In Crest Hill, a 30-year-old village that lies just north of Joliet and 37 miles southwest of Chicago's Loop, the tornadoes tore roofs and walls from buildings in three housing complexes.

Six to seven people were killed there, their bodies found in cornfields nearby, according to Crest Hill Police Lt. Jim Ariagno and Lockport Fire Chief Dave Martis.

In nearby Plainfield, Plainfield High School and Grand Prairie Elementary School were leveled. Nearby St. Mary Immaculate Catholic Church and its gymnasium, rectory and school building were gutted.

Two people were reported killed at the church school, including the principal, Sister Mary Keenan. At least three people were reported dead in Plainfield High School.

In Crest Hill, where dozens of residents awaited news about relatives, searchers were marking houses with a large, yellow 'X' when it was determined that all occupants had been accounted for.

After dark, parents continued to look for missing children as searchers used dogs, floodlights, cranes and helicopters to continue their search for survivors and bodies.

Following the storm, the Red Cross and other disaster agencies set up emergency shelters for those displaced by the storm, and local hospitals scurried to care for the hundreds of injured.

The tornadoes first touched down in Oswego, in Kendall County, but did only minor damage there, according to Mark Finn, a spokesman for the Illinois State Police.

Plainfield and Crest Hill are communities populated by both suburban commuters who work in Chicago and by others who work at industrial facilities in the area.

Numerous houses in Plainfield's Peerless Estates, mostly wooden bungalows and ranches, were splintered and tumbled into foundations or onto lawns.

Peter Kalantzis, 17, said he saw the twister peel the roof from a house. "The whole roof, all intact, landed in the middle of the road," he said.

The tornadoes continued southeasterly, paralleling U.S. 30, and crossed I-55, toppling trees and blowing the cars across the road.

At Plainfield High School, many found it remarkable that more people were not injured.

Wayne DeSutter, football coach at Plainfield, said 102 varsity football players had been on the practice field and were herded into the school's gymnasium at the first signs of the storm. Minutes later, the twisters roared across the field as the players huddled against an interior wall that remained intact as other parts of the gym collapsed.

"Our ears started popping and we heard a loud crash," one of the football players told Associated Press. "The rest of our school's gone. I went outside and looked. It doesn't look like our town anymore."

"We had some very short warnings. We immediately took cover," teacher Steve Schernerhorn said.

The storms struck with almost no warning, although the area had been under a severe thunderstorm alert calling for the possibility of tornadoes. When one tornado carved a five-mile path in the Plainfield area, civil defense sirens had just sounded a warning in part of the area.

Principal Jim Waldorf said as many as 175 people were in the high school at the time of the storm, and a number suffered minor injuries.

Teacher Mike Salisburg said some cars in the school parking lot were thrown as far as several hundred yards.

Alice Stone, 33, an assistant superintendent of finance for the Plainfield school district, said that about 25 people were working in the building as the tornadoes approached. Several employees rushed to a records vault in the center of the building, although several did not make it inside.

Classes, which were scheduled to begin with a half-day Wednesday, were canceled through the rest of the week, according to school superintendent Roland Smith.

Guy Allton, 16, watched the storm from his house near the school. "We went outside, and there was picnic tables flying past our faces and stuff," Allton said.

Afterward, he helped pull people from nearby houses, he said, including two badly injured babies.

"Everything was messed up. Everything on James Street is totaled," Allton told AP. "Lots and lots of people hurt."