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CHICAGO — Just as Chicagoans were settling into dinner, and the White Sox were in the middle of a doubleheader on the South Side Tuesday, a Russian Air Force jet flew over Chicago at high altitude, likely collecting reconnaissance along the way.

The flights are allowed under the Treaty on Open Skies, which permits member nations to conduct pre-arranged aerial surveillance over each other’s territory. The flights are meant to give member nations the opportunity to confirm each other are abiding by the rules of military treaties. Rules require the planes to be unarmed but they do contain high tech surveillance and monitoring equipment.

Radar imagery provided to WGN by FlightRadar24 shows the Tupolev jet departed Dayton, Ohio, Tuesday in route to Great Falls, Montana. The plane was flying at roughly 36,000 feet as it approached the Indiana-Illinois border and made a sharp turn north sending it directly over Chicago and just west of Milwaukee. Naval Station Great Lakes is located along that route in Chicago’s northern suburbs. It’s the Navy’s largest training installation and home to the Navy’s only Boot Camp.

In April, a different Russian spy plane reportedly flew over two of America’s top nuclear laboratories and other strategic sites.

A Russian military spokesperson said: “U.S. specialists on board will monitor the use of surveillance equipment and compliance with the provisions of the agreement.” The Russians have also used the treaty to take a look at key strategic sites along the East Coast of the United States.

A specially equipped U.S. Air Force jet took part in a similar observation flight over Russian territory in February. A Pentagon spokesperson said at the time that six Russian observers travelled with the U.S. crew, per treaty procedures.

Why did the Russians fly over Chicago? A U.S. Military spokesperson did not immediately respond to a request for comment.

One aviation expert suggested spying may not have been the main motivator of the routing.

“It may have been avoiding storms,” said Ian Petchenik of FlightRadar24.

Check out the video below of the same Russian Federation Air Force Tupolev Tu-154M on a similar mission in Puerto Rico earlier this year: