Hackers believed to be affiliated with Russia’s military breached the Ukrainian gas company where former Vice President Joe Biden’s son had served on the board as it became a focus of the impeachment inquiry into President Trump, according to a U.S. cybersecurity firm.

Attempts to hack into Burisma Holdings began last November as Congress was holding hearings on whether Mr. Trump abused his office by pressuring his Ukrainian counterpart to work with his personal lawyer Rudy Giuliani to investigate Mr. Biden and his son Hunter, according to research published Monday by Area 1, a California-based company.

The hacking attempts are ongoing and linked to the Russian military intelligence unit previously known as the GRU, which hacked and leaked Democratic emails during the 2016 presidential election, Area 1 said.

The Russian Embassy in Washington didn’t immediately respond to a request for comment. Russia has denied hacking into U.S. elections.

It wasn’t known what information the hackers were seeking or what they obtained, the firm said. In at least one instance, the hackers tricked the recipient of a phishing email into sharing login credentials that allowed them access into one of Burisma’s servers, the company said. Area 1’s findings were earlier reported by the New York Times.....

Oren Falkowitz, co-founder of Area 1 and a former National Security Agency official, said the company was “100%” confident that Russia’s military intelligence was responsible for the breach. Mr. Falkowitz said that the company had made a series of disclosures about its findings to U.S. intelligence agencies before its publication of the research Monday. Area 1 wasn’t hired to conduct the research.

Mr. Falkowitz said the company discovered evidence of a Russian phishing campaign on New Year’s Eve and determined in the following days that each of the targets were subsidiaries of Burisma.