Few details were immediately released, but CBS reports multiple injuries. A suspect with a high-powered rifle was shot and secured by cops at about 11:30 a.m. Chicago time, authorities told CBS.
"I heard popping and everybody dropped to the ground," evacuated passenger Robert Perez told CBS.
Los Angeles Police are on citywide tactical alert.
Terminal 3 serves JetBlue and Spirit airlines. Some inbound flights from Chicago airports are being diverted in air at this hour and other flights out of Chicago are reportedly delayed.