VIDEO: How Russia Used Social Media to Meddle in the 2016 Election and Undermine U.S. Democracy

In a way it was the perfect crime: using American-made social media tools to subvert American democracy.

Prior to testifying before congressional committees last month, legal representatives from Facebook, Twitter and Google -- all headquartered in the Bay Area -- revealed that an ongoing Russian-connected disinformation campaign that used their platforms to try to smear Hillary Clinton, manipulate political opinion and generally inflame tensions was significantly larger in scope (and presumably impact) than initially reported.

As many as 126 million Facebook users over the last two years may have viewed posts clandestinely produced and circulated by Russian operatives, the social media giant reported. Twitter, meanwhile, acknowledged it had identified 2,752 accounts controlled by Russians, and found that roughly 36,000 Russian bots had tweeted 1.4 million times in the runup to the election in an effort to generate trending hashtags. And Google said it had found more than 1,110 YouTube videos, amounting to about 43 hours of content, that were related to the Russian subversion efforts.

Russian operatives also purchased more than $100,000 of targeted ads on the platforms, the tech companies revealed.

In his Oct. 31 testimony before a separate House intelligence committee, Facebook general counsel Colin Stretch said: "They were able to develop a significant following for a relatively small amount of money. It was undertaken by people I think who understand social media. These people were not amateurs."

Democrats on the House Intelligence Committee released a large sampling of the Russian-linked ads, event notifications, memes, pages and petitions that had been widely shared on Facebook, Twitter and Instagram before and after the election.  See some of the original posts here.

As this related Above the Noise video explains, much of the issue comes down to online security and the increasing challenge of knowing the source of your online information and protecting yourself from being scammed.

The Russian-related messages in question were primarily posted from the social media accounts of fabricated individuals and organizations. Much of the content attacked Hillary Clinton and the Democratic Party, often promulgating wildly unfounded conspiracy theories.

For example, one ad placed in July 2016 by a made-up Facebook user called "Being Patriotic" showed a picture of Clinton's face covered with a black X, and read: "Hillary Clinton is the co-author of Obama's anti-police and anti-Constitutional propaganda." Targeted at Trump supporters in New York, it urged them to attend an upcoming rally.

The ad has since been traced back to a Kremlin-backed group in St. Petersburg, Russia, that paid Facebook for it in Russian rubles for the equivalent of about $250, according to data Facebook provided to the House committee. As many as 15,255 Facebook users saw the ad in their news feed and 1,312 clicked on it.

Another particularly colorful one equated Clinton to Satan.

RELATED: The U.S. Has Its Own Long History of Meddling in Foreign Elections [Timeline]

Many of the made-up Russian posts, tweets and videos didn't actually mention either candidate, but were rather efforts to stir up and further widen deep-seated divisions on a wide range of explosive political issues, including immigration, racial justice and religion. The ads targeted specific groups of people in strategic voting locations. The Russian-backed propaganda effort also publicized or financed at least 60 different controversial events around the country related to some of the most politically polarizing issues.

Some posts received only limited visibility; other were viewed hundreds of thousands of times.

One Russian-authored fake Facebook group called Blacktivist urged people to rise up and retaliate against police violence, espousing “an eye for an eye." Another group, targeting white conservatives, insisted that Black Lives Matter activists should “be immediately shot.”

Another Facebook ad from "Secured Borders" had an image of a sign on what looked like the southwest border that read: "No Invaders Allowed." Facebook reported that the ad was purchased in Russian rubles, for about $1,600, and was viewed by as many as 97,000 users.

U.S. intelligence agencies have long known “with high confidence” that Russia's "influence campaign" attempted to undermine U.S. democracy and help Trump beat Clinton in the election. But not until these most recent disclosures was the sprawling extent of Russian social media infiltration fully understood.

As Scott Shane of the New York Times wrote in September (before the full extent of the impact had been revealed):

"The Russian information attack on the election did not stop with the hacking and leaking of Democratic emails or the fire hose of stories, true, false and in between, that battered Mrs. Clinton on Russian outlets like RT and Sputnik. Far less splashy, and far more difficult to trace, was Russia’s experimentation on Facebook and Twitter, the American companies that essentially invented the tools of social media and, in this case, did not stop them from being turned into engines of deception and propaganda."

Sen. Dianne Feinstein (D-Calif), a member of the Senate panel, gave the three social media companies a stern warning that they needed to be more vigilant at stopping illicit foreign use of their platforms — or Congress would be forced to get involved.

"You bear this responsibility. You created these platforms and now they are being misused," she said. "You have to be the ones to do something about it or we will."

WATCH: What teenagers should know about bots, produced by Jacqueline Kao of the PBS Newshour Student Reporting Lab program at San Mateo High School.