Nearly 120,000 units of digital currency bitcoin worth about US$72 million was stolen from the exchange platform Bitfinex in Hong Kong, making it the second-biggest security breach ever of such an exchange.
Bitfinex is one of the largest exchanges for bitcoin, and is known in the digital currency community for having a platform that has deep liquidity in the U.S. dollar/bitcoin currency pair.
Zane Tackett, Director of Community & Product Development for Bitfinex, told Reuters on Wednesday that 119,756 bitcoin had been stolen from users' accounts and that the exchange had not yet decided how to address customer losses.
"The bitcoin was stolen from users' segregated wallets," he said.
Bitcoin plunged just over 23 percent on Tuesday after the news broke. On Wednesday it was up 1 percent at $545.20 on the BitStamp platform.
Tackett added that the breach did not "expose any weaknesses in the security of a blockchain", the technology that generates and processes bitcoin, a web-based "cryptocurrency" that can move across the globe anonymously without the need for a central authority.
The volume stolen amounts to about 0.75 percent of all bitcoin in circulation.
"It's the biggest USD exchange, so outside China it's the one that everyone has an account with," said Antony Lewis, a bitcoin expert in Singapore. "It's very liquid, folk can trade on margin, lots of daily volume."
It is not yet clear whether the theft was an inside job or whether hackers were able to gain access to the system externally.
Bitfinex suspended trading on Tuesday after it discovered the breach. It said on its website that it was investigating and cooperating with the authorities.
The attack on Bitfinex was reminiscent of a similar but larger breach at MtGox, a Tokyo-based bitcoin exchange that was forced to file for bankruptcy in early 2014 after hackers stole an estimated $650 million worth of customers' bitcoins.
"We are investigating the breach to determine what happened, but we know that some of our users have had their bitcoins stolen," the company said.
"We are undertaking a review to determine which users have been affected by the breach. While we conduct this initial investigation and secure our environment, bitfinex.com will be taken down and the maintenance page will be left up."
The company said it has reported the theft to law enforcement. It said it has not yet determined the value of digital currencies stolen from customer accounts.
Bitfinex also said as it goes through individual customer losses, it may need to settle open margin positions, associated financing, or collateral affected by the security breach.
Any settlements will be at the current market price as of 18:00 UTC (1800 GMT), the company said.
The attack on Bitfinex was reminiscent of a similar breach at Mt. Gox, a Tokyo-based bitcoin exchange forced to file for bankruptcy in early 2014 after hackers stole an estimated $650 million worth of customer bitcoins.
Bitcoin late on Tuesday was down 6.35 percent at $567.83 on the BitStamp platform.