A Southwest Airlines plane that skidded off a snowy runway and onto a highway, killing a child traveling in a car, was a relatively new aircraft with no history of problems, the airline said on Friday.

US government safety investigators and Southwest's own team were at Midway Airport trying to determine what caused the Boeing 737 with 98 passengers and five crew to plunge through a fence-like barrier and onto a busy street during a heavy snowstorm Thursday evening.

The airport, which is southwest of downtown Chicago, was hit with more than 9 inches of snow before and after the accident but it was not known if that played a part in the mishap. Visibility was one-quarter to one-half mile with snow falling when the flight from Baltimore made an instrument landing, the airline said.

"Weather will be a factor that will be thoroughly reviewed," said Ellen Conners, a member of the National Transportation Safety Board. But she told a briefing at the airport that it was still early in the investigation and no conclusions had been reached.

She also said the aircraft would remain in place on the street at least until Saturday.

After the incident, the passengers were evacuated safely from the plane. The airline said there were 13 injuries in all, the most severe involving passengers in cars struck by the plane as it came to a halt. Joshua Woods, 6, of Leroy, Indiana, was killed when the car in which he was traveling was crushed.

His two younger brothers and parents were also injured. Three people were still being treated for injuries on Friday but were expected to survive.

Gary Kelly, Southwest's chief executive, told a news conference the carrier took delivery of the aircraft when it was new in July of 2004, and said mechanics completed a routine maintenance check the day before the accident.

"There were no indications that the aircraft was experiencing any kind of maintenance troubles. The captain... has been flying for Southwest for 10 years... The first officer has been flying for Southwest for 2 years (and) this was the first flight of day for both pilots," he said.

At least two vehicles collided with the plane, said Chicago Fire Commissioner Cortez Trotter. "One of the cars was under the side of the plane, under one of the wings. The other was near the nose of the plane," Trotter said.

A witness, Marie Valasquez, told FOX-TV she heard "a loud screeching noise. I thought it was a semi (truck) that had crashed and didn't expect to see the Southwest plane out there."

The FAA said the plane landed on a runway which is roughly 6,500 feet (1,981 metres) long and among the longest at the compact airport, which is surrounded by roads and residential areas. Midway was closed after the accident but reopened on Friday.

Trotter said some fuel had spilled but was quickly contained by emergency crews.

A similar accident in March 2000 involved another Southwest jet that ran off the runway at Burbank Airport in California, ran through a fence and came to rest on a street.

Thursday's accident happened on the 33rd anniversary of a crash near Midway that took 45 lives, including a member of the US Congress, George Collins.