(CNN) -- United Airlines says a computer outage that caused the cancellation of nine flights and delayed 580 others was the result of an equipment failure.
"United's operations are running normally today following yesterday's network outage," said a statement sent to CNN Wednesday by airline spokeswoman Mary Ryan.
During the more than two-hour-long outage Tuesday, United's website redirected to a message that said, "We are experiencing technical difficulties. This will temporarily affect our ability to serve you online."
At Chicago's O'Hare International Airport, a United hub, check-in kiosks were dark and passengers stood in long lines. At the gates, people boarded planes while United agents checked them off on a hand-drawn seating chart.
United passengers in San Francisco recounted being unable to get boarding passes, and some Newark passengers took to social media to share pictures of long lines and handwritten boarding passes they say they were given.
"The outage was caused when a piece of communication equipment in one of our data centers failed and disabled communications with our airports and website," the airline said. "We have fully redundant systems and we are working with the manufacturers to determine why the backup equipment did not work as it was supposed to."
Passengers on affected flights can cancel or rebook their flights without penalty, according an earlier statement from the airline. "United apologizes for the disruption caused to travelers at affected airports and is re-accommodating customers as quickly as possible."
CNN's Elizabeth Chmurak contributed to this report.