Drudge has the word 'explosion'.
I hadn't heard that before, have you?
From Drudge
FLASH: Fire has been going for 1.5 hours... not contained.... LaSalle Bank Building, 135 South Lasalle in the Loop... Large explosion seen on WLS ... fire moving from one window to another... WLS TV reporting confusion in the stairwells...doors locking... 'firefighters say go back up.....but doors locked'... people shown on CBS WBBM injured in lobby... MORE...