CHICAGO (AP) -- United Airlines said a piece of computer hardware was behind the technology meltdown that delayed 580 flights and shut down its website for more than two hours.

The outage on Tuesday prevented workers from boarding passengers on time. United passengers reported long lines in the airline's hubs in Newark, N.J., San Francisco, Houston, and Chicago. It also caused nine cancellations.

The problem was a piece of hardware in a data center that failed to communicate properly with other computer equipment, said Megan McCarthy, a spokeswoman for United Continental Holdings Inc. A backup system failed to take over for the troubled hardware.

United has been struggling with technology problems since March, when it switched to a passenger information computer system that was previously used by Continental. United and Continental merged in 2010. That system, called "Shares," has needed extensive reworking since March to make it easier for workers to use.

McCarthy said that Tuesday's problems were not caused by Shares.

United's computer problems came on the same day that airlines already had to deal with Hurricane Isaac, which has disrupted travel in cities around the Gulf of Mexico.

Shares of the Chicago-based company fell 6 cents to $18.46 in afternoon trading.