Facebook admitted this month that it recommended content produced by Russian operatives to some users, the latest acknowledgment that it underestimated how extensively foreign actors manipulated its platform around the time of the 2016 presidential election.

“This happened in some cases,” Facebook told the Senate Intelligence Committee, in a written follow-up to a November hearing about election interference through social media. “Because we were not aware that these Pages were not legitimate, they were sometimes recommended when people followed similar Pages, for example.”

The answer was in response to a question from Senator Susan Collins, R-Maine, who asked if Facebook’s recommendation engine suggested “to any Facebook user that they view, follow, or join any of the Russian-linked pages.” The letter, dated Jan. 8, was made public on Thursday.

Facebook has been under fire for its failure to either recognize or acknowledge how much Russians were using its service to push propaganda to potential voters. CEO Mark Zuckerberg originally called that a “crazy” idea, before apologizing last year. Facebook then said that 10 million people saw Russian-bought political ads, but later said that 126 million Americans may have seen such content.

Facebook also told the Senate in the response that it has found “insignificant overlap” between Russian-produced pages and ads and those created by the election campaign of President Donald Trump.

In answering Collins’s question about whether there was evidence of potential collusion between Trump and Russian group on social media, Facebook said it “does not believe it is in a position to substantiate or disprove” such allegations.

Facebook was also asked if it made money from content produced by the Internet Research Agency, or IRA, a Russian propaganda farm.

“Ads generally did not run on IRA Pages, and we expect that any revenue from such ads would be immaterial,” the company said.

Facebook said that Russian groups also used the platform to promote real-world events. There were a total of 129 events created on 13 IRA pages that were viewed by around 338,300 accounts. About 62,500 accounts said they intended to go to an event.

The company plans to “significantly expand” the number of people working on election integrity before the 2018 mid-term elections later this year, and said it will be hiring “people who investigate this specific kind of abuse by foreign actors.”