British Airways apologized on Friday after the credit card details of hundreds of thousands of its customers were stolen over a two-week period in the most serious attack on its website and app.

The airline discovered on Wednesday that bookings made between Aug. 21 and Sept. 5 had been infiltrated in a “very sophisticated, malicious criminal” attack, BA Chairman and Chief Executive Alex Cruz said. It immediately contacted customers when the extent of the breach became clear.

Around 380,000 card payments were compromised, the airline said, with hackers obtaining names, street and email addresses, credit card numbers, expiry dates and security codes – sufficient information to steal from accounts.

The attack came 15 months after the carrier suffered a massive computer system failure at London’s Heathrow airport, which stranded 75,000 customers over a holiday weekend.

Shares in BA’s parent, International Airlines Group , were down 2 percent in afternoon trading on Friday.

Cruz said the carrier was “deeply sorry” for the disruption caused by the attack which was unprecedented in the more than 20 years that BA had operated online.

He said the attackers had not broken the airline’s encryption but did not explain exactly how they had obtained the customer information.

“There were other methods, very sophisticated efforts, by criminals in obtaining the data,” he told BBC radio.

IT security company Avast said that based on the limited information available the attackers had probably targeted a gateway between the airline and a payment processor because no travel details had been stolen.

“Quite often, when it’s just a hack of a database somewhere it is hard to identify when something has been compromised,” Avast’s consumer security expert Pete Turner said.

“This feels much more like a transaction-type attack, where data is moving about within the system.”

Compensation

Britain’s government said authorities including the National Cyber Security Centre and the National Crime Agency, part of the country’s police, were piecing together what happened.

“Specialist officers from the NCA’s National Cyber Crime Unit are managing the ongoing investigation and are on site working with BA to gain a better understanding of the incident,” the NCA said.

The country’s Information Commissioner’s Office said it had been alerted by BA and it was making enquiries. Under new GDPR data regulations companies must inform regulators of a cyber attack within 72 hours.

BA advised customers to contact their bank or credit card provider and follow their recommended advice. It also took out ads in national newspapers on Friday.

Cruz said anyone who lost out financially would be compensated by the airline.

Data security expert Trevor Reschke said that like any website which sees large volumes of card transactions, BA was a ripe target for hackers.

“It is now a race between British Airways and the criminal underground,” said Reschke, head of threat intelligence at Trusted Knight.

“One will be figuring out which cards have been compromised and alerting victims, whilst the other will be trying to abuse them while they are still fresh.”

NatWest, one of Britain’s biggest card issuers, said it was receiving higher-than-usual call volumes because of the breach.

It said in a recorded message that its security systems would likely stop any fraud as a result of the hack but anyone affected should look out for unusual activity on their accounts.

American Express said clients did not need to take any action and the company would alert anyone with unusual activity on their cards.

IAG said the data breach had been resolved and the website was working normally, and that no travel or passport details were stolen.

After the computer system failure in May 2017, BA said it would take steps to ensure such an incident never happened again, but in July it was forced to cancel and delay flights out of the same airport due to problems with a supplier’s IT systems. (Reporting by Paul Sandle and James Davey in London and Sangameswaran S and Rama Venkat Raman in Bengaluru Editing by Keith Weir and Louise Heavens)