CHICAGO (AP) — A tornado outbreak that swept the Chicago area this week produced at least 11 twisters, and preliminary findings show they were relatively weak, the National Weather Service said Friday.

No injuries were reported when Wednesday's storms ripped roofs from buildings and toppled trees in the Chicago area and points west. One tornado touched down near O’Hare International Airport, sending passengers dashing for shelter and disrupting hundreds of flights.

Weather service staff who surveyed the storm damage tracks have confirmed that at least 11 tornadoes touched down Wednesday evening, the agency's Chicago-area office said in an update. The agency said it would continue investigating the outbreak over the next few days.

The 11 twisters were spawned by rotating thunderstorms known as supercells, the weather service said.

Eight of the confirmed tornadoes had preliminary ratings of EF-0, which produce winds of 65-85 mph (105-137 kph), while the three others had preliminary ratings of EF-1, which have winds of 86-110 mph (138-177 kph), it said.

Over the years, many tornadoes have struck the Chicago area, and several have hit within the limits of the nation's third largest city, the weather service said.

An outbreak this spring produced 22 tornadoes in Chicago's expansive metro area, tying the record for the most in the region in a single calendar day, or a 24-hour span, the weather service said. The 22-tornado record was previously set during a June 30, 2014, outbreak.

The weather service's tornado records for the Chicago area date to 1950.