CHICAGO - A preliminary report into two near collisions midair at Chicago's O'Hare International Airport raises questions about air-traffic control at one of the world's busiest airports.

The report released Monday by the National Transportation Safety Board doesn't draw conclusions or assess blame. But it provides a sometimes unsettling picture.

Controllers, pilots and others interviewed after a near miss August 8, 2011, mentioned an obstruction in the middle of the air-traffic control room that sometimes forced controllers to yell vital information across the room.

Those interviewed about a May 16, 2011, incident said a controller may have been so distracted, he forgot one plane had been given the OK to land, so he gave another the OK to take off.

In each instance, planes came within several hundred feet of each other.

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