As speculation mounts about whether President Trump’s digital team helped Russia target voters with fake news during the 2016 election, the campaign’s former digital director Brad Parscale has agreed to an interview with the House Intelligence Committee. But if investigators really want to get to the bottom of how foreign propagandists tried to sway voters, the answer may lie within Facebook's servers.

Parscale tweeted a statement about his decision to meet with committee members Friday morning, defending the work his San Antonio firm Giles-Parscale did for the campaign. Parscale rejected the notion that his team shared data with Russian operatives to help them target receptive voters. In his statement, Parscale says the campaign “used the exact same digital marketing strategies that are used everyday by corporate America.” And he specifically points out how closely the campaign worked with Facebook, Google, and Twitter.

The campaign had designated liaisons from both Facebook and Google working inside Parscale's San Antonio-based office, who were intimately involved in the inner workings of the digital and data team, according to Parscale's statement. They helped carry out an effort of great scale and sophistication. During the campaign, the Trump campaign ran up to 50,000 variants of its Facebook ads a day, learning which ones resonated best with voters. It also deployed so-called “dark posts,” non-public paid posts that only appear in the News Feeds of the people the advertiser chooses.

Parscale has credited that collaboration with delivering Trump's victory. "Facebook and Twitter were the reason we won this thing," Parscale told WIRED shortly after the election. "Twitter for Mr. Trump. And Facebook for fundraising."

To be sure, there is nothing unusual about this arrangement. Large advertisers working with social media companies are often assigned designated representatives. "Someone from Facebook or Twitter’s ad sales will call you and be your account manager," says Adam Sharp, who ran Twitter's government and elections team until December 2016.

Which is why investigators exploring the Russian social-media operation may learn as much from platforms like Facebook as from the Trump campaign. Congress could subpoena the company for data on which entities made large scale ad buys—the kind that can actually help swing an election. Facebook keeps lists of who it extends lines of credit to, though of course, those lists would only reflect the agency doing the advertising and not the many entities that might be funding the ads. Facebook also allows political advertisers to upload their own voter lists for targeting purposes. Investigators could ask the company whether any advertisers used duplicate lists to disseminate pro-Trump or anti-Clinton ads. That could indicate a coordinated effort by some outsider to influence the election on Trump's behalf, though it's possible that data is inaccessible because of the way Facebook hashes the information in its system.