Following the indictment of 13 Russian nationals accused of breaking U.S. laws by interfering with the 2016 presidential election, Facebook said it has “more to do” and vowed to better protect its social media platform from outside intrusion.

The indictments, announced Friday by the U.S. Department of Justice, highlight how the Russian nationals waged a campaign of information warfare against the U.S., including relying on social media accounts to pose as people living in the country. The agency also alleges that those indicted began buying ads on Facebook in August that promoted a post accusing Hillary Clinton of voter fraud.

“We know we have more to do to prevent against future attacks,” said Joel Kaplan, vice president of global policy at Facebook, in a statement. “We’re making significant investments, including increasing the number of people working on security from 10,000 to 20,000 this year.”

Kaplan said the federal indictments confirm the company’s announcement last year that “foreign actors conducted a coordinated and sustained effort to attack our democracy.”

He added, “As we said publicly last year, this kind of foreign interference violates all of our values. These indictments now say it also violated the law.”

Facebook and other social media networks have come under fire for failing to take action against efforts aimed at disrupting or influencing U.S. elections, among other issues. The indictments didn’t include an allegation that the defendants’ efforts impacted the outcome of the election.

But Deputy Attorney General Rod Rosenstein said the indictment is a reminder that “people are not always who they appear to be.”