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CHICAGO, Illinois (CNN) -- Investigators are probing an accident involving a Southwest Airlines jet that skidded off a runway at Chicago's Midway Airport, plowed into an intersection and killed a boy.
The 6-year-old was in the car with his parents and two younger brothers, including an infant, said Deborah Song, spokeswoman for Advocate Christ Medical Center in Oaklawn.
Their car was crushed under the plane's nose and fuselage, said Fire Department spokesman Larry Langford. (Watch him describe victims and scene -- 2:22)
The mother, in her 30s, was in good condition; the father, in his 40s, was in serious condition; the 4-year-old middle son was in fair condition; and the infant was in serious condition, Song said.
Four people in the second car, which was under a wing of the Boeing 737-700 jet, were in serious but stable condition.
Midway, which lies in a dense residential and commercial district of the city, west of downtown, was closed almost immediately and was not set to reopen until Friday morning, the FAA said.
Flight 1248 was arriving from Baltimore, Maryland on Thursday evening with 98 passengers and five crew members onboard. (Watch footage from the scene -- 1:33)
Chicago's acting commissioner of aviation, Pat Harney, said airport runways had been salted and cleared throughout the day, but said he didn't know when the runway where the jet landed was last serviced.
Southwest Airlines CEO Gary Kelly said there were no indications of maintenance problems with the plane, and that it underwent a routine check Wednesday in Phoenix.
"All indications are that the aircraft was cleared to land properly," Kelly said. He said that despite the heavy snow and light winds, there was "plenty of visibility."
The pilot of the jet has been with Southwest 10 years, Kelly said, and the co-pilot about three years.
Passenger: 'It was really bumpy'
It had been snowing all day in Chicago and visibility was poor at the time of the landing, shortly after 7 p.m.
The National Weather Service had issued a heavy snow warning in the city and surrounding area.
There were approximately 8 inches of snow on the ground by early evening, and winds were blowing at between 13 mph and 18 mph.
Stanley Den, who was on the plane, said he noticed the plane wasn't slowing down during the landing.
"It looked like the runway wasn't plowed at all. The runway looked just like grass. I couldn't really tell if we were on the runway or the grass. It was really bumpy. We were kind of going for a while until the impact when we hit, maybe I guess, a barrier fence, went through that and into the middle of the street with cars and stuff riding past us," he said.
Resident: Two loud booms
Amanda Doherty lives near the airport and said she went to the scene shortly after the crash and saw a car pinned under the airplane with its headlights still on. (Watch witness account -- :27 )
A bartender at a nearby pub said he heard two loud booms when the plane crashed into the intersection of 55th Street and Central Ave, on the northwest side of the airport.
"We thought it was an automobile accident and we looked out the window and we saw the tail section of a Southwest airliner laying across the street," said Tom Fitzgerald, adding that he saw passengers exiting the rear of the plane.
"People were running and ambulances were coming down the street."
Video from the scene showed at least 10 ambulances at the scene and dozens of fire trucks and other emergency vehicles converging on the intersection. The plane's fuselage was bent and its nose was on the ground.
The accident occurred the same day as a deadly crash at Midway 33 years ago, when a plane coming in for a landing struck several homes before plowing into one. Forty-five people -- 43 passengers and two in the house -- were killed.