An online “impersonator” of a Democratic National Committee (DNC) staffer tried to contact presidential campaigns, including Sen. Bernie SandersBernie SandersWinners and losers from the South Carolina debate Five takeaways from the Democratic debate Sanders most searched, most tweeted about candidate during Democratic debate MORE’s (I-Vt.) campaign, the committee said in a statement to the candidates Wednesday.

Bob Lord, the DNC’s chief security officer, wrote in an email to the campaigns that “adversaries will often try to impersonate real people on a campaign,” The Associated Press reported. 

He added that the “adversaries” could try to get campaign workers to “download suspicious files, or click on a link to a phishing site” or set up calls or in-person meetings to record and release.

Lord warned that the “impersonator” contacted the Sanders campaign and at least two others and had a domain registered overseas. But he acknowledged that anyone can register a domain name in any country.  

“Attribution is notoriously hard,” he wrote. “The appropriate authorities have been alerted.”

“If you are using an alternate domain, please refrain from doing so and let us know if you are operating from a domain that others have not corresponded with before,” Lord added. “Do not use your personal mail account for official business.”

Sanders campaign spokesman Mike Casca confirmed the incident with the AP and said the domain was registered in Russia.

“It’s clear the efforts and investments made by the DNC and all the campaigns to shore up our cybersecurity systems are working,” Casca said, according to the AP. “We will remain vigilant and continue to learn from each incident.”

The Hill reached out to the DNC and the Sanders campaign for confirmation.

The Vermont senator said on Friday that he was briefed about a month ago that Russia was attempting to boost support for his campaign.

Democratic campaigns have been cautious about cybersecurity since Hillary ClintonHillary Diane Rodham ClintonDemocratic insiders stay on the sidelines in 2020 race Hillicon Valley: Twitter falling short on pledge to verify primary candidates | Barr vows to make surveillance reforms after watchdog report | DHS cyber chief focused on 2020 The Hill’s Campaign Report: High stakes at last Democratic debate before Super Tuesday MORE campaign chairman John Podesta’s emails were hacked and published after he received an email seemingly from Google directing him to change his account.