The IT shutdown which led to chaos for British Airways was caused by the "uncontrolled return of power" following an outage, the airline has said.
Servers at its data centre Boadicea House, near Heathrow, were physically damaged by the malfunction.
Around 75,000 passengers were disrupted as flights were cancelled following the incident on Saturday morning.
The carrier was unable to resume a full schedule until Tuesday and many passengers who had already checked in when the issue emerged are still waiting to be reunited with their luggage.
BA was accused of greed after the GMB union suggested the issue could have been prevented if the airline had not cut "hundreds of dedicated and loyal" IT staff and contracted the work to India last year.
The cause of the initial power outage and the subsequent surge has not yet been revealed.
The airline said in a statement: "There was a loss of power to the UK data centre which was compounded by the uncontrolled return of power which caused a power surge taking out our IT systems. So we know what happened we just need to find out why.
"It was not an IT failure and had nothing to do with outsourcing of IT, it was an electrical power supply which was interrupted.
"We are undertaking an exhaustive investigation to find out the exact circumstances and most importantly ensure that this can never happen again."
Experts predict BA is facing huge compensation costs, with reports suggesting the bill could top £100m (€115m).
The airline said it will "comply with all of the relevant EU compensation regulations", including welfare claims such as hotel accommodation, transport to and from hotels, meals and telephone calls.
It has added extra staff to its customer relations department to help process payments.
All of the delayed bags have been processed at Heathrow and are on their way to customers around the world.
The airline accepted that "it may take some time to complete the process", particularly for passengers on complex itineraries or in locations it does not serve every day.
BA's parent company, IAG, saw shares initially fall by around 4% in the first day of trading in London after the outage occurred.
On Saturday night, travellers spent the night sleeping on yoga mats spread on terminal floors after BA cancelled all flights leaving Heathrow and Gatwick.
BA chief executive Alex Cruz said the outsourcing of jobs was not to blame for the "catastrophic" power failure.
The airline said it has a freephone number - 0800 727 800 - for people affected and customers are advised to check ba.com.