The FAA's warning notice also accused the Chicago Department of Aviation of making several false statements in its own internal inspections.
"The daily self-inspection records do not reflect actual conditions in the field, violations have not been noted on the self-inspection records that are evident in the field," reported the FAA warning, according to the Tribune.
Among the safety violations at O'Hare, inspectors found debris on runways, including rocks, garbage and wooden survey stakes. The debris is a major violation to regulations requiring runways to be completely clear of hazards to avoid interference with planes. The FAA also reported tall grass and weeds, which attract birds and wildlife, growing near runways.
The report also found that airport workers were under trained in several cases. In one example, workers were found driving on runways before completing the required training.
The Department of Aviation has already corrected many of the violations, and will have addressed all the issues by November, according to Karen Pride, a spokeswoman. She added that the Department takes the FAA report very seriously.
"The highest priority we have at the airport is safety and the cleanliness of the runways and the taxiways," she said.