CHICAGO (CBS) — For the sixth time in the past week, rock-throwers have damaged cars on a Chicago area expressway.

The latest incident happened around 9:30 p.m. Wednesday on the Stevenson Expressway. Police say people were throwing rocks from an embankment near the railroad overpass at Pulaski Road.

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But the suspects had already taken off by the time officers arrived on the scene.

At least 12 vehicles were damaged, with cracked windshields or dented hoods. No one was injured.

This is at least the sixth incident in the past week in which rock-throwers have damaged cars on area expressways.

On Monday around 1:15 p.m., a white Toyota sport-utility vehicle ended up with a broken windshield when someone threw a rock from the top of the Pulaski Road overpass on the Eisenhower Expressway.

Less than four hours later, witnesses saw “a few kids” throwing rocks at cars on Interstate 57 from a bridge near 151st Street in the southern suburbs.

Five cars were damaged in the same area Sunday night, when rocks were also dropped from I-57, the Sun-Times Media Wire reported. That incident happened around 8 p.m. Sunday on northbound I-57 near 147th Street.

On Saturday, at least six vehicles were damaged when someone threw rocks at cars from a the Burlington-Northern Santa Fe railroad overpass on the Stevenson, between Kedzie Avenue and Pulaski Road.

The cars pulled over after going under the overpass, when their windshields, hoods or sunroofs were damaged by rock tossed from the overpass, the wire reported.

In the most serious incident on Wednesday of last week, the Chicago Skyway had to be shut down for two hours due to a rock-throwing incident. The hoods and windshields of several cars were damaged by projectiles on the Skyway near 105th Street.

Police originally believed that snipers were firing at passing cars, but no bullets or shell casings were found.

The day after the incident, two boys, ages 14 and 15, were charged as juveniles with one count of felony vehicular endangerment and six counts of misdemeanor criminal damage to property.