The boss of British Airways' parent company says that human error caused an IT meltdown that led to travel chaos for 75,000 passengers.
Willie Walsh, chief executive of IAG, said an engineer disconnected a power supply, with the major damage caused by a surge when it was reconnected.
He said there would now be an independent investigation "to learn from the experience".
However, some experts say that blaming a power surge is too simplistic.
Mr Walsh, appearing at an annual airline industry conference in Mexico on Monday, said: "It's very clear to me that you can make a mistake in disconnecting the power.
"It's difficult for me to understand how to make a mistake in reconnecting the power," he said.
He told reporters that the engineer was authorised to be in the data centre, but was not authorised "to do what he did".
IAG has commissioned an "independent company to conduct a full investigation" into the IT crash and is "happy to disclose details" of its findings, Mr Walsh said. The name of the company involved had not been disclosed.
The BBC reported last week that senior company executives at IAG were pushing for an external probe into the computer meltdown.
BA had said that a power surge caused the computer problem, but gave little further explanation.
However, an email leaked to the media last week suggested that a contractor doing maintenance work inadvertently switched off the power supply.
The email said: "This resulted in the total immediate loss of power to the facility, bypassing the backup generators and batteries... After a few minutes of this shutdown, it was turned back on in an unplanned and uncontrolled fashion, which created physical damage to the systems and significantly exacerbated the problem."
But the BBC's transport correspondent, Richard Westcott, has spoken to IT experts who are sceptical that a power surge could wreak such havoc on the data centres.
BA has two data centres about a kilometre apart. There are question marks over whether a power surge could hit both. Also, there should be fail-safes in place, our correspondent said.
BA has already launched its own internal investigation, led by its chief executive Alex Cruz.
Following the computer crash, which caused travel chaos for people travelling from Heathrow and Gatwick, Mr Walsh gave his full backing to Mr Cruz.
BA and IAG also rejected claims that the incident was due to Mr Cruz's decision to outsource the airline's IT department to India as part of cost-cutting measures.
Also on Monday, Mr Walsh apologised again for the incident, saying: "When you see customers who suffered, you wouldn't want it to happen to any airline or any business."
He added: "I wouldn't suggest for one minute we got communications right at BA, we didn't."