An electrical company in the US state of Vermont says it has found malware code allegedly used by Russian hackers on one of its company laptops.
The Burlington Electric Department said it had taken "immediate action to isolate" the computer, which was not connected to the electrical grid.
The government alerted them to the "Grizzly Steppe" code on Thursday.
The same day, the US expelled 35 Russian diplomats over alleged Russian interference in November's election.
Moscow has denied any involvement in the hacking of the Democratic party and Hillary Clinton's presidential campaign.
On Friday, US President-elect Donald Trump praised Russian President Vladimir Putin for not expelling American diplomats in a tit-for-tat response.
The Burlington company said it was "working with federal officials to trace this malware and prevent any other attempts to infiltrate utility systems".
It has also briefed state officials and vowed to fully support the investigation.
The statement followed a Washington Post report, citing unnamed US officials, that Russian hackers had penetrated the electrical facility, underlining the vulnerability of America's national grid.
The newspaper later retracted the claim that hackers had accessed the grid, saying the computer hacked was not attached to the system.
According to the news website: "The incursion may have been designed to disrupt the utility's operations or as a test to see whether they could penetrate a portion of the grid."
'Systemic, relentless, predatory'
Politicians in Vermont, including the Democrat Governor, Peter Shumlin, are calling for a full investigation into the incident.
"Vermonters and all Americans should be both alarmed and outraged that one of the world's leading thugs, [Russian President] Vladimir Putin, has been attempting to hack our electric grid, which we rely upon to support our quality-of-life, economy, health, and safety," he said in a statement.
Democratic congressman Peter Welch said the discovery showed "how rampant Russian hacking is".
"It's systemic, relentless, predatory," he added.
US officials believe Russia was behind the hacking of Ukraine's electrical grid in December 2015, which plunged parts of the country into darkness and left about 225,000 people without power.
Experts said it was the first known power outage caused by a cyber attack.
Update 25 January 2017: This story has been updated to make clear that the Washington Post has since retracted the claim in its article that hackers had accessed the US national grid.