This weekend was one of the worst in recorded history for people who were traveling by plane in the U.S., especially if they were supposed to take off or land somewhere on the East Coast.

The numbers have not yet been finalized, but according to FlightAware.com data, more than 5,000 flights over the weekend were late while more than 650 were cancelled altogether. At the Baltimore-Washington International airport, around 70 percent of flights were disrupted while at the Ronald Reagan Washington National Airport that percentage was over 70 percent. Dulles International Airport was hit the worst, with almost 90 percent of flights being delayed or cancelled on Saturday.

It goes without saying that people want to know what happened and who is to blame.

Well, at this point, the most likely culprit seems to be a software upgrade which was done at one of the traffic control centers at Leesburg, Virginia. A malfunction occurred when the system which was installed five months ago was being upgraded.

Laura Brown, spokeswoman for the FAA has announced that air traffic is working normally today and that the software was returned to earlier version, which seems to have solved the problem.