A Southwest Airlines jet that skidded off a runway at Midway International Airport and into a city street needed about 800 more feet of runway to come to a safe stop, federal investigators said Thursday.

The Dec. 8 accident killed a 6-year-old boy inside one of two vehicles crushed by the plane when it crashed through a fence at the airport and plowed into the street.

A preliminary investigation by the National Transportation Safety Board shows the airplane touched down with about 4,500 feet of runway remaining but that snowy conditions and other factors meant the plane needed about 5,300 feet of runway to stop.

The jet's actual stopping distance was about 5,000 feet, the NTSB said. A tail wind also contributed to the accident because it caused the plane to land faster than normal.

Investigators said last week that the plane's reverse thrusters, which should have slowed the jetliner, didn't immediately kick in when the pilots tried to deploy them.

The flight's captain also told investigators that the plane didn't decelerate normally so he applied manual braking. When the first officer also noticed "poor braking effectiveness," he moved his seat forward to apply maximum braking, the NTSB said in a written report.

Both said they applied maximum pressure to the brakes as the airplane skidded off the runway and came to a stop in the street.

Air traffic controllers said the runway's conditions were fair for most of the runway and poor at the end, according to the NTSB.