The Treasury Department was hacked as part of the large-scale Russian campaign that still has the federal government reeling, Treasury Secretary Steven Mnuchin said Monday.
Speaking to CNBC, Mnuchin said "our unclassified systems did have some access" from hackers.
"At this point, we do not see any break in into our classified systems," he said.
"There's been no damage, nor have we seen any large amounts of information displaced," he added later.
The Treasury Department is at least the third federal agency, along with the Commerce and Energy departments, to acknowledge that it was hacked as part of a far-reaching espionage campaign widely believed to be the work of Russian intelligence.
Sen. Ron Wyden, D-Ore., a member of the Intelligence and Finance Committees, said Monday night in an emailed statement that Treasury staff said the breach began in July and that there is still much they don't know.
"Finance Committee staff was briefed today on the SolarWinds hack, and I appreciate Treasury and IRS officials sharing what they know as they continue to learn more," he said. "According to IRS, there is no evidence that IRS was compromised or taxpayer data was affected. However, the hack of the Treasury Department appears to be significant."
"Microsoft notified the agency that dozens of email accounts were compromised," Wyden said. "Additionally the hackers broke into systems in the Departmental Offices division of Treasury, home to the department’s highest-ranking officials. Treasury still does not know all of the actions taken by hackers, or precisely what information was stolen."
"That would amount to a goldmine for foreign adversaries looking to spy on or blackmail government officials," said Wyden, who said he expected to be briefed on the specifics of the hack soon.
Government officials have widely tied the hacks to Russia's SVR intelligence agency since the news broke this month. Secretary of State Mike Pompeo said Friday that the Russians were "pretty clearly" behind the operation, and Attorney General William Barr told reporters that it "certainly appears to be" Russia's work.
Only President Donald Trump has cast doubt on the connection, tweeting Saturday that the media are "petrified of discussing the possibility that it may be China (it may!)" and inexplicably tying the hack to conspiracy theories to explain how he lost the election.
More agencies have been reported to be investigating whether they were affected, reflecting the potential scope of the campaign.
The hackers began by compromising a third-party information technology company in Austin, Texas, called SolarWinds in March, affecting an untold number of its tens of thousands of customers.
That gave Russia a wide range of potential targets. Microsoft alone said last week that it had contacted more than 40 organizations that had been compromised.