Story highlights Airline says it will operate full schedule at London airport

IT crash on Saturday affected 170 airports worldwide

London (CNN) British Airways said its IT systems are back up and a full flight schedule is planned Tuesday at London's airports, where thousands of passengers were stranded because of the breakdown.

While the carrier toiled to find out exactly what went wrong, it apologized and said it was working to reunite travelers and their bags.

"We are extremely sorry for the frustration and inconvenience customers experienced over the Bank Holiday weekend and thank them for their patience and understanding," British Airways said in a statement late Monday.

The airline earlier said that it does not have "a complete picture" of what caused the catastrophic IT outage that took down its systems in 170 airports across 70 different countries.

The impact of the incident, which began on Saturday, continued with a third day of disruption on Monday, a national holiday in the UK. At least 75,000 customers have been affected.

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