Facebook has shut down a sophisticated disinformation operation on its platform that engaged in divisive messaging ahead of the U.S. midterm elections, the company said Tuesday, an escalation of what a top executive described as an “arms race” to manipulate the public using its tools.

Facebook said it discovered 32 false pages and profiles that were created between March 2017 and this May, which lured 290,000 people with ads, events and regular posts on topics such as race, fascism and feminism — and sought to stir opposition to President Trump. The company informed law enforcement before it deleted the profiles Tuesday morning. It also notified lawmakers of the activity this week, and said it would notify the real Facebook users who were swept up in the operation.

One of the most popular pages had links to the Internet Research Agency (IRA), the Kremlin-backed organization of Russian operatives that flooded Facebook with disinformation around the 2016 election, Facebook said. Yet the operators of the newly banned pages, whom Facebook said it was not in a position to identify, were more clever about covering their tracks. Lawmakers and experts were quick to attribute the activity to Russia.

The disclosure — the first admission of coordinated disinformation on Facebook that could affect the November election — is a sign that manipulation continues to be an active problem for Facebook and its billions of users, even after the company has spent heavily to prevent it. It also raised questions about whether other technology companies are still being used as conduits for disinformation, as Google, Twitter and others were around the last election.

Google didn’t respond to queries about whether it has observed any new coordinated disinformation. Twitter declined to comment.

Both Facebook and disinformation operators have become more savvy in the last year, since the company trickled out information about the IRA activity. Facebook, which detected the most recent pages through manual investigations, artificial intelligence and leads from law enforcement, has taken a more aggressive approach to rooting out and disclosing political abuse.

The deceptive accounts have also modified their tactics to mask their identities. They used third parties to buy ads on their behalf and didn’t use Russian Internet protocol addresses or pay with Russian rubles, according to Facebook. And they relied heavily on promoting events, which Facebook doesn’t scrutinize as closely as ads on political or social issues. One such event was organized to counterprotest a far-right rally planned in Washington this month.

“We’re sharing this today because of the timing of the event that was planned for Washington,” said Facebook Chief Operating Officer Sheryl Sandberg, who described the fight against disinformation as an “arms race” on a call with reporters Tuesday. She said the investigation was in an early stage but that she wanted to warn the public because the pages were promoting the counterprotest.

“It is clear that much more work needs to be done before the midterm elections to harden our defenses, because foreign bad actors are using the exact same playbook they used in 2016 — dividing us along political and ideological lines, to the detriment of our cherished democratic system,” said Rep. Adam B. Schiff (Calif.), the top Democrat on the House Intelligence Committee.

The new revelations could worsen Facebook’s political headaches in Washington, where the Senate Intelligence Committee is set to hold a hearing Wednesday on efforts to spread political discord on social media ahead of the 2018 midterms. In September, the congressional panel aims to hold another hearing with representatives from Facebook, Google and Twitter, aides have said.

Facebook last year revealed that the IRA had operated 470 accounts and pages on its main platform as well as its sister site, Instagram. Some of the content first surfaced in June 2015, and Russian-generated propaganda may have been seen by 126 million people, the company told the Senate. Some of the ads had been paid for in rubles, and some of the Internet addresses were traceable back to Russia, where the IRA operated out of St. Petersburg. In February, special counsel Robert S. Mueller III indicted 13 Russians for election interference, based on their manipulation of Facebook, Twitter and other social media.

Facebook continued to sweep its site for further disinformation campaigns. In June, the company told The Washington Post that it hadn’t detected any Russian or IRA activity related to the midterms.

Then last week, in a call discussing election integrity, the company appeared to hint that something had surfaced when it said it couldn’t discuss anything related to law enforcement investigations. Facebook executives said they discovered the pages in the past two weeks.

In recent weeks, leaders in the administration, including Director of National Intelligence Daniel Coats, have said that active campaigns by Russia were taking place on social media. Trump has gone back and forth on whether Russia is still seeking to interfere in U.S. elections. The White House did not immediately respond to a request for comment.

Department of Homeland Security chief Kirstjen Nielsen on Tuesday applauded Facebook’s action. “This threat is very real, and Americans need to know that,” she said during an interview on Fox News Channel’s “The Daily Briefing.”

Two clues tied the IRA to the 32 new accounts. Facebook found that one of the most popular pages, “Resisters,” briefly had a co-administrator — for seven minutes — that was a known IRA account before the co-administrator disappeared, according to Facebook. Another known IRA account had previously shared an event associated with the same page.

Some of the Facebook pages existed as far back as March of last year, suggesting Facebook had missed some accounts in its initial sweep, which took place starting in early 2017 and continued through the fall. Facebook publicly named only four of the pages, suggesting that its understanding of the disinformation is still unfolding. It gave one research organization access to eight pages.

In total, the operatives put out 9,500 Facebook posts. Other most-followed pages were “Aztlan Warriors,” “Black Elevation,” “Mindful Being” and “Ancestral Wisdom.” The most popular page had 18,000 followers — and the least popular, 16 followers. The 32 accounts ran 150 ads, which cost $11,000 and were paid for in U.S. and Canadian dollars.

“Adversaries are not going to make sloppy mistakes,” said Renee DiResta, an expert on disinformation and research director at New Knowledge, a cybersecurity start-up. She called the new disinformation campaigns “increasingly complex.”

Notably, they promoted 30 events, which can be more significant than ads because they seep into real life. The largest of the events, which Facebook hasn’t yet identified, had 4,700 accounts interested in attending, and 1,400 people registered to attend, Facebook said. The Black Elevation page also offered to hire people to promote their events, according to the files that Facebook released Tuesday.

One Resisters event scheduled to take place last November in New York’s Times Square was called “Trump Nightmare Must End.” Four thousand Facebook users said they were interested in attending or would attend, according to images Facebook shared. It’s unclear if the event took place.

Resisters, which described itself as a feminist group on Facebook written as reSisters, recently promoted an event called “No Unite the Right 2 DC.” The event, which was slated for August 10-12, was intended as a counterprotest to a planned far-right rally organized by supporters of the Charlottesville rally last summer. The counterprotest event had 2,600 people who said they were interested and 600 who said they would attend. Inauthentic administrators of the resisters page connected with administrators from five legitimate pages to co-host the event. Facebook said it notified those latter administrators.

In the past year, Facebook has hired thousands of new security staff, partnered with research organizations, and improved its artificial intelligence tools for detecting disinformation.

Facebook on Tuesday said its approach was to focus on forensics rather than make political statements about foreign governments. The issue of attribution to Russia has been a point of contention among executives.

Facebook executives previously complained that law enforcement had not provided them with leads about Russian threats, but agencies said their hands were tied because of laws that prohibit domestic surveillance. In May, Facebook hosted a meeting with senior law enforcement officials to discuss the midterms, a sign that the relationship was getting warmer, The Washington Post reported.

Graham Brookie, director of the Atlantic Council’s Digital Forensic Research Lab, which received an advance copy of some of the pages from Facebook, said in a Medium post that, though it was too early to attribute the origin of the pages, “an initial scan shows behavioral patterns and use of language reminiscent of the troll operations run from Russia in 2014 through 2017.”

Similar to the IRA pages around the election, the latest crop of pages were designed to build loyalty within a community through a shared identity that can later be manipulated. The Aztlan Warriors and Ancestral Wisdom pages focused on racial pride and anti-colonial messages for black, Hispanic and Native Americans, he pointed out.

The latest pages, Brookie said, were more targeted to influence the current election cycle because “midterms tend to be more activist elections.”