The hackers who tried to infiltrate Sen. Claire McCaskill’s staff email network left behind a trail of evidence that bolsters experts’ assessment that they were from the Russian group responsible for the 2016 cyberattacks on the Democratic National Committee.

Staffers for the Missouri Democrat were targeted earlier this year by a password phishing scam. Hackers sent staffers an email telling them they needed to create new passwords for their Microsoft Exchange email accounts and directed them to a webpage that mimicked the U.S. Senate’s Active Directory Federation Services (ADFS) log-in page, the Daily Beast reported in July.

The attack ultimately was unsuccessful. Analysts concluded that Russia’s GRU intelligence agency directed it.

“Russia continues to engage in cyber warfare against our democracy. I will continue to speak out and press to hold them accountable,” McCaskill said at the time of the Daily Beast report. “While this attack was not successful, it is outrageous that they think they can get away with this. I will not be intimidated. I’ve said it before and I will say it again, Putin is a thug and a bully.”

New outside research shows how the experts who traced the attack back to GRU came to their conclusion, TechCrunch reported.