The Russian hackers who penetrated United States government agencies broke into the email system used by the Treasury Department’s most senior leadership, a Democratic member of the Senate Finance Committee said on Monday, the first detail of how deeply Moscow burrowed into the Trump administration’s networks…

The Treasury Department ranks among the most highly protected corners of the government because of its responsibility for market-moving economic decisions, communications with the Federal Reserve and economic sanctions against adversaries. Mr. Wyden said the hackers had gained access to the email system by manipulating internal software keys.

The department learned of the breach not from any of the government agencies whose job is to protect against cyberattacks, but from Microsoft, which runs much of Treasury’s communications software, Mr. Wyden said. He said that “dozens of email accounts were compromised,” apparently including in what is called the departmental offices division, where the most senior officials operate.

“Treasury still does not know all of the actions taken by hackers, or precisely what information was stolen,” he said.