Did the U.S. Government really get a warning that Pearl Harbor would be attacked 3 months before it happened?

I have been igned to do a report on Pearl Harbor. I have heard lots of rumors that the feds broke the Japaneese code and knew about the attack but let it happen anyway just so they would have an excuse to jump into the war. We were already helping the Brits by lending naval warships, and the Chineese had American Mercenary pilots fighting the Japaneese (the Flying Tigers). The 3 month warning is probably why there were no Aircraft Carriers in the harbor at the time of the attack. This allegation is particurlarly troublesome to me because over 2000 of our servicemen died there.