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Snap quietly reviewed its sales records around the 2016 U.S. presidential election to determine if Russian-backed entities purchased political ads through its platform — a probe that didn’t turn up any red flags.

But other tech companies — including Yahoo and Reddit — either have not scanned their own data or aren’t sharing their detailed findings publicly, even at a time when federal lawmakers are aggressively probing whether Kremlin-backed agents spread misinformation through the country’s most prominent websites last year.

Through its corporate parent, the Verizon-owned Oath, Yahoo outright declined to answer any questions from Recode this week. Yahoo earned roughly $3.01 billion in digital ad revenue worldwide digital ad spending in 2016, according to eMarketer.

For the moment, at least, it’s only Facebook, Google and Twitter that find themselves in the crosshairs of the U.S. Congress. They’ve been asked to brief lawmakers in private about the ads they may have sold during the election to Russian-backed entities, and they face the risk of additional, public scrutiny if they are forced to testify at two hearings in the House and Senate in the coming weeks.

But the probe on Capitol Hill is only just beginning. And it could easily expand if federal lawmakers sense that Russia — in an attempt to infiltrate the U.S. presidential election and trouble other elections to come — have targeted their efforts well beyond Facebook, Google and Twitter.

“The Committee’s investigation is ongoing and will engage with technology companies as it sees fit,” said a spokeswoman for Sen. Richard Burr, the leader of the Senate Intelligence Committee.

Other key lawmakers on the panel and its House counterpart did not immediately respond to requests seeking comment.

So far, Facebook has found 470 accounts tied to Russian agents. Those accounts purchased about 3,000 ads — valued at around $100,000, and viewed by about 10 million U.S. users — that aimed to stoke social and political tensions ahead of Election Day. Relying on Facebook’s data, Twitter subsequently discovered about 200 accounts connected to Kremlin interests.

And Google has not yet finished its own internal probe, sources have said, which presumably focuses on both its ad business as well as YouTube. It’s expected to brief House and Senate investigators in the coming weeks.

The potential for similar scrutiny already looms over Reddit. Sen. Mark Warner, the top Democrat on the Senate Intelligence Committee, previously has flagged the message board-style site for the conspiracy-minded — and hate-tinged — content that appeared there last year.

For now, at least, Reddit said it has “not been contacted by Senate investigative offices so [we] are not able to speak to any specific concerns they may have.” And as a general rule, a spokeswoman added, Reddit conducts “regular policy reviews” and vets all ads it sells manually.

But the company did not respond to additional questions from Recode as to whether it performed an exhaustive audit after the 2016 presidential election. Nor did Reddit say if it studied whether its own users — many of whom use Reddit anonymously — have deliberately sought to share misinformation on behalf of the Russian government.

While Facebook has acknowledged Russian efforts on its site, meanwhile, it hasn’t yet addressed questions about potential Kremlin-aligned attempts to compromise Instagram. A spokesman did not respond to emails seeking comment this week.

Previously, though, Facebook has pledged that its internal review is ongoing, admitting that it could still turn up additional evidence that Russia sought to spread misinformation on its platforms.

And at Snap, digital advertising amounted to a significant portion of its $404 million in revenues last year. But the company said this week it had less to fear about Russian meddling because of the kind of ads its offers in the first place.

Tony Romm is the senior editor for policy and politics at Recode. He can be reached at tony@recode.net, or on Signal, Confide, iMessage and WhatsApp at 215.779.9597.

From consumer brands to the average wedding-goer, users can purchase photo filters tied to a specific geographical area. Snap only allowed those ads to be purchased online beginning in February 2016, and they are still reviewed to see if they comply with internal rules — especially if they are political in nature. They also have to make clear who purchased them. And only more established partners can buy the short videos that auto-play in between the snaps that users post for all of their followers to see. These video Snap ads have been sold by salespeople since 2015.

“Every ad purchased on Snapchat during the 2016 U.S. presidential election was reviewed by human eyes,” a spokeswoman for Snap stressed to Recode. “We're still digging deeper, but our preliminary review of our 2016 sales records has produced no evidence that any Russian entities purchased political ads on Snapchat."

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