They were Chicago Department of Aviation security officers. Interestingly, the Chicago PD (which was not involved in the incident) released a statement saying the guy fell and injured himself!
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...a Chicago Police news affairs officer — not the aviation cops — initially released a statement to an unnamed media outlet saying that a “69-year-old male Asian airline passenger” became “irate” aboard the flight and that aviation officers “attempted to carry the individual off the flight when he fell.
http://chicago.suntimes.com/news/video-appears-to-show-passenger-being-removed-from-united-flight/
“Interestingly, the Chicago PD (which was not involved in the incident) released a statement saying the guy fell and injured himself!”
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Well, it is CHICAGO, after all. Could have been worse, he might have accidentally fell and hurt himself as the PD were trying to suit the guy up with cement boots.