Having trouble towards the end...nowhere to go?

Just wondering if anyone can help out? I am on the section towards the end of game after you beat Xavier's champions...you go back to the mansion, and then you end up at this facility where they manufacture the sentinels. You have to beat 4 tanks, which I did, and then one of the guards drops a key, i picked it up, but there is no where to go with it? Any ideas on this one? Thank you