A 6-year-old boy in one of the vehicles was killed, authorities said. He was among eight people hurt on the ground. Two passengers on the plane suffered minor injuries, Aviation Department spokeswoman Wendy Abrams said.

Southwest Airlines Flight 1248 from Baltimore was landing at Midway International Airport with 98 passengers and five crew around 7:15 Thursday night when it slid through the fence.

The airport area had 7 inches of snow, but Abrams said runway conditions at the time were acceptable.

The landing seemed normal at first, passenger Larry Vazzano said.

"There was a bump. I saw snow rush over the wing, then there was a big bump," said Vazzano, 54, of Baltimore. "I braced myself on the seat in front of me."

Mike Abate, 35, of suburban Milwaukee, said he could see from the plane that a man was carrying an injured child and that other people were being taken away in an ambulance.

"We were safe on the plane," Abate said. "The toughest part was to realize that someone was under the belly of the plane."

The nose of the plane was crushed in the accident and a severely damaged engine was on the ground, Chicago Fire Department spokesman Larry Langford said.

He said some passengers used inflatable slides to get out of the plane in the blowing snow, while others used stairs at the rear of the plane. The airport was shut down, with no estimate when it would reopen.

All the injured on the ground were in the two vehicles, officials said. The 6-year-old was dead when he arrived at a hospital; two adults and two other children were hospitalized, their conditions ranging from serious to good, said Advocate Christ Medical Center spokeswoman Deborah Song.

The Boeing 737 slid through the northwest corner of the airport, through the boundary fence and into the road, according to the Federal Aviation Administration's regional office in Chicago. Langford said at least two vehicles were damaged, and one was pinned under the plane.

Midway, Chicago's second largest after O'Hare International, is closely bordered by streets lined with homes and businesses. It serves more than 17 million travelers a year, many of them on Southwest.

National Transportation Safety Board and FAA officials from Washington were on their way to Chicago to investigate.

Southwest CEO Gary Kelly said the plane had circled Midway for 30 to 35 minutes because of the weather and the flight traffic before it was cleared for landing on the airport's 6,500-foot runway.

He would not speculate on what could have caused the accident but said the plane's captain had been flying for about 10 years.

"There are no indications that there are any maintenance problems with that aircraft whatsoever," Kelly said. He said the plane had just had a service check in Phoenix on Wednesday.

While Abrams didn't immediately fault runway conditions, James Burnett, a former NTSB chairman, said the weather would be a focus of the investigation.

"When you're looking at a runway overrun, it almost always involves a runway condition that's improper," Burnett told WFLD-TV. "But that's not the only thing."

Standard procedure also calls for pilots to be tested immediately after a crash for alcohol in their blood, FAA spokeswoman Laura Brown said in Washington, D.C.

Southwest flies an all-737 fleet with over 400 aircraft.

Snow caused troubles for travelers across the Midwest on Thursday, with as much as 10 inches on the ground in some areas.

The accident occurred 33 years to the day after a crash at Midway that killed 45 people, two of them on the ground.

In that crash, a United Airlines jet struck tree branches about a mile from the airport, then hit the roofs of a number of bungalows before plowing into a home, bursting into flames. Eighteen passengers survived.