Sen. Ron Wyden, D-Ore., provided new details of the hack following a briefing to Senate Finance Committee staff by the IRS and Treasury Department.
Wyden said that though there is no indication that taxpayer data was compromised, the hack “appears to be significant,” including through the compromise of dozens of email accounts and access to the Departmental Offices division of the Treasury Department, which the senator said was home to its highest-ranking officials.
A Treasury Department spokeswoman declined to comment on Wyden's statement.
Microsoft notified the Treasury Department that dozens of email accounts were compromised, Wyden said.
President Donald Trump sought to downplay the severity of the hack last week, tweeting without any evidence that perhaps China was responsible.