CHICAGO (AP) - Confusion descended on downtown Thursday when a series of mechanical problems darkened hundreds of office buildings, snarled traffic and skidded the Chicago Board of Trade to a halt. Thousands of commuters left early for the day when power was lost, pushing the rush hour forward by hours. Police struggled to keep cars moving through darkened intersections. Extra buses were called in to handle the crush and generators were set up in Union Station, which handles Amtrak and suburban commuter trains. Board of Trade officials closed the markets at 1 p.m. after they were warned by utility officials that power was going to be shut down, said board spokesman Bret Gallaway.