Updated 8:48 p.m. | Sen. Claire McCaskill said Thursday that Russian hackers did not successfully penetrate her Senate office’s computer systems, following a report that one of her policy aides was the target of a phishing attack. 

“While this attack was not successful, it is outrageous that they think they can get away with this. I will not be intimidated,” the Missouri Democrat said in a statement. “I’ve said it before and I will say it again, [Russian President Vladimir] Putin is a thug and a bully.”

The Daily Beast reported Thursday evening that the email address for McCaskill’s aide was uncovered as part of its investigation into congressional candidates targeted by hacking attempts. A Microsoft official revealed last week that the company had defended three candidates against such attacks.

The hacking attempt reportedly came in the form of a phishing attack, in which the target would receive an email to change his or her password, leading the target to a malicious site that mirrored the legitimate Senate login page. The Daily Beast reported that the tactic was similar to one successfully implemented by Russian hackers when they hacked into the Democratic National Committee in 2016.

McCaskill said last year that she was concerned about cyberattacks targeting her campaign. She is one of the most vulnerable incumbent senators up for re-election this year, running in a state that President Donald Trump carried by 19 points. Inside Elections with Nathan L. Gonzales rates the race a Toss-up.

At the time, she said her campaign was taking steps to address this threat, but she would not disclose those details.

Senate Intelligence Vice Chairman Mark Warner said the news confirmed what he and others have warned for a while.

“The Russians saw 2016 as a success, and they’ll be back in 2018, unless we do far more to protect ourselves than we’re currently doing,” the Virginia Democrats said in a statement.

Warner criticized the Trump administration for underplaying the threat from Russia as lawmakers wrangle with how to curb its attempts to interfere in U.S. elections.

“Unfortunately, the lack of leadership from the White House means that we still have no all-of-government approach to addressing this threat,” he said.

Griffin Connolly contributed to this report.