US Crypto company Harmony said that hackers stole about $100 million in digital coins from one of its key products.

The theft occurred from its Horizon “bridge,” a special software tool for transferring crypto between different blockchains, which are the underlying software used to support different cryptocurrencies such as Bitcoin and Ether.

Harmoney specializes in developing such tools for DeFi, or decentralized finance sites. DeFi sites offer loans and other bank-type services without the traditional gatekeepers such as banks.

Bridges are a particularly vulnerable piece of software, and are heavily targeted by thieves as a result. Bridges have had over $1 billion stolen from them just in 2022, according to blockchain analytics firm Elliptic. In March, in a single heist, North Korean hackers got away with $615 million worth of crypto from Ronin Bridge, which is used to transfer crypto into and out of the game Axie Infinity.

Harmony said on Twitter it was, “working with national authorities and forensic specialists to identify the culprit and retrieve the stolen funds.”

Elliptic said, based on publicly-available blockchain data, it appears the thieves got away with  ether, Tether, and USD Coin, which they later swapped for ether on DeFi exchanges.