The theft occurred Thursday morning, per a tweet thread posted by the Harmony team. An address linked in the thread holds 85,837.251 ETH, worth approximately $98 million as of press time.
"We have begun working with national authorities and forensic specialists to identify the culprit and retrieve the stolen funds," the team said in a message, further writing:
"We have also notified exchanges and stopped the Horizon bridge to prevent further transactions. The team is all hands on deck as investigations continue. We will keep everyone up-to-date as we investigate this further and obtain more information.
The Harmony devs also said that its bitcoin bridge was unaffected, saying that related assets "stored on decentralized vaults are safe at this time."
Cross-chain bridges have come under attack in the post. Earlier this year, a bridge connecting Axie Infinity's Ronin sidechain with the Ethereum network was exploited, resulting in the loss of more than $600 million in crypto. The Horizon bridge is another such blockchain-to-Ethereum connection.