BOSTON (AP) — A U.S. cybersecurity company says Russian military agents have successfully hacked the Ukrainian gas company at the center of the scandal that led to President Donald Trump's impeachment.

Russian agents launched a phishing campaign in early November to steal the login credentials of employees of Burisma Holdings, the gas company, according to Area 1 Security, a Silicon Valley company that specializes in e-mail security.

Hunter Biden, son of former U.S. vice president and Democratic presidential hopeful Joe Biden, previously served on Burisma's board.

It was not clear what the hackers were looking for or may have obtained, said Area 1's CEO, Oren Falkowitz, who called the findings “incontrovertible” and posted an eight-page report. But the timing of the operation suggests that the Russian agents could be searching for material that damaging to the Bidens.

The House of Representatives impeached Trump in December for abusing the power of his office by enlisting the Ukrainian government to investigate Biden, a political rival, ahead of the 2020 election. A second charge accused Trump of obstructing a congressional investigation into the matter.

“Our report doesn't make any claims as to what the intent of the hackers were, what they might have been looking for, what they are going to do with their success. We just point out that this is a campaign that's going on,” said Falkowitz, a former National Security Agency offensive hacker whose company's clients include candidates for U.S. federal elected offices. In an earlier interview, he told The Associated Press that top candidates for the U.S. presidency and House and Senate races in 2020 have in the past few months each been targeted by about a thousand phishing emails.

Falkowitz did not name the candidates. Nor would he name any clients.

Russian hackers from the same military intelligence unit that Area 1 said was behind the operation targeting Burisma have been indicted for hacking emails from the Democratic National Committee and the chairman of Hillary Clinton's campaign during the 2016 presidential race.