DALLAS - The Southwest Airlines Co. jet that skidded off a snowy runway and into a Chicago street Thursday night, killing a 6-year-old boy in a car, showed no mechanical problems during a check the day before the accident, Chief Executive Gary Kelly said.

The Boeing jet was equipped with anti-skid and automatic braking systems, but like other Southwest jets, the braking system was turned off to let pilots manually control the plane, Kelly said Friday.

Kelly said at a news conference at Southwest headquarters that it was too early to say what caused the accident, which occurred during a snowstorm that reduced visibility at Midway Airport to between one-quarter and one-half mile.

It was the first fatal accident in Southwest's 35-year history.

"It's a sad day at Southwest, and we're going to focus all our effort on taking care of our passengers and their families, and supporting the NTSB in the investigation," Kelly said. The National Transportation Safety Board began its investigation within hours of the accident.

Kelly said 13 people were taken to hospitals and three remained hospitalized Friday morning. He had no information on their conditions.

The flight from Baltimore-Washington International Airport to Chicago was the first of the day for the pilot, a 10-year Southwest veteran, and the co-pilot, Kelly said. The airline has not disclosed the names of the crew or passengers.

Southwest took delivery of the Boeing 737-700 in July 2004. Southwest operates only 737s, and the 700 is the latest update to that model.

It comes with an automatic braking system, but Southwest doesn't use it, Kelly said.

"We let our flight crews make the determination to brake the aircraft," he said. "It's just a choice, and we think that it's been obviously very, very successful over a long period of time."

Kelly said there was no way to know until the investigation is done whether the auto-braking system would have prevented Thursday night's accident.

The plane had undergone a routine maintenance check Wednesday in Phoenix.

"There were no indications that the aircraft was experiencing any kind of maintenance problems," Kelly said.

Southwest has no immediate plans to change operations at Midway, one of the carrier's largest operations, with 196 daily departures. Southwest had operated there for 20 years without a previous incident, Kelly said.

The accident raised questions about Southwest's operations at smaller airports, some of which, like Midway, are surrounding by houses and commercial buildings.

The main runway at Midway is 6,500 feet. Southwest also operates at Burbank, Calif., which has an even shorter runway, 5,800 feet, and was the site of a similar accident in which a Southwest jet slid through an airport fence. No one was seriously injured in that accident.

Some Dallas residents have raised safety arguments against Southwest's plans to expand long-haul flights from Love Field. Kelly said Thursday's accident should have no bearing on the debate in Congress over expanding Love Field.

"They are obviously different airports with different runways and different configurations," he said. The main runway at Love Field is 8,800 feet, he said.

Kelly and nearly 100 Southwest employees were headed to Chicago to meet with passengers and family members of those involved in the accident.

Colleen Barrett, Southwest's president, was planning to contact the family of the boy who was killed, said airline spokeswoman Ginger Hardage. Kelly said Southwest family "is grieving this loss and our thoughts and prayers go out to the child's family."