By Josh Smith SEOUL (Reuters) – North Korean hackers are most likely behind an attack last week that stole as much as $100 million in cryptocurrency from a U.S. company, three digital investigative firms have concluded. The cryptoassets were stolen on June 23 from Horizon Bridge, a service operated by the Harmony blockchain that allows assets to be transferred to other blockchains. Since then, activity by the hackers suggests they may be linked to North Korea, which experts say is among the most prolific cyber attackers. U.N. sanctions monitors says Pyongyang uses the stolen funds to support i…