Facebook might've thought it was out of the woods when it presented its internal investigation of the platform's role in the 2016 Brexit vote. But the UK's House of Commons blasted the report as barely skimming the surface, and told the social media giant to have a second look.

On Wednesday, Facebook responded, promising to delve deeper into the murky operations of the St. Petersburg troll factory to see if “there was coordinated activity similar to that which was found in the U.S.” But experts say that no matter how detailed the new inquiry will be, too much time has passed and the U.K government will be left with more questions than answers.

“I suspect that [Facebook] will see only a certain aspect of the campaign, because the troll factory is getting better and better at hiding its traces,” said Ben Nimmo, senior fellow at the Atlantic Council’s Digital Forensic Research Lab.

According to Facebook’s initial investigation, just 200 people in the U.K. saw adverts paid for by the St. Petersburg-based Internet Research Agency, better known as Russia’s notorious troll factory, with the ads costing just $0.97 to deliver.

But a report by the Committee on Foreign Relations in the Senate, published earlier this month, challenged the adequacy of Facebook’s original investigation, saying its limited focus on the Internet Research Agency means it missed other troll operations.

Nimmo said it was already clear Moscow’s goal was “amplifying the Leave campaign” and that focusing narrowly on the official actions of Russia’s troll factory misses the bigger picture, which includes a sophisticated and highly coordinated campaign involving numerous fake accounts.

"The problem is that adverts are obvious, but in this debate, they are pretty irrelevant," Nimmo said. “The much more important question is what were the masquerade accounts doing, and that is much harder for Facebook to do, and is going to take much longer.”

The bigger picture

Trying to get a complete picture of exactly what happened, 18 months after the vote took place, is no small task, even for a company as sophisticated as Facebook.

The original investigation focused solely on ads that were officially paid for by the Internet Research Agency, but these constitute just a fraction of the unofficial operations being conducted by this organization, analysts said, and an even smaller fraction of the entire Russian disinformation machine.

“The Internet Research Agency is not a lone wolf," wrote Diana Pilipenko, the principal investigator for the Moscow Project at American Progress, last November. "It is one of many Russian entities engaging in trolling."

If the Brexit operation bears any similarities to the Kremlin’s U.S. election influence gig, these fake stories did not come with a label saying "This is brought to you from a troll factory in St. Petersburg" but came from accounts masquerading as the unofficial Tennessee GOP account or a gun rights group in Texas.

In fact, the troll factory is known to have run thousands of these fake accounts and groups across various social media platforms.

During its investigation into the Russian influence campaign during the U.S. presidential election, Facebook used official IRA accounts as a starting point to identify the unofficial troll accounts that were run from the same building.

But analysts fear that if Facebook takes a similar approach with the Brexit vote investigation, they’ll have a much harder time, because the Russian operatives have grown much better at hiding their tracks.

Hiding their tracks

One of the ways Facebook was able to identify the spread of fake accounts in the U.S. was through the email addresses or phone numbers used to set up profiles that linked back to Russia.

But, in early 2017, the troll factory accounts changed their phone numbers to burner phone numbers obtained online that did not have Russian phone codes, Nimmo said. They also switched from using Russian email accounts to Gmail or Hotmail accounts, making it much more difficult for Facebook to track them.

A former employee of the troll factory confirmed this new strategy to outlet Dozhd last year, and said that the agency operates a separate ‘‘Facebook desk’’ to fight back against the social network’s efforts to delete its most sophisticated fake accounts.

Another method Facebook can use to root out false accounts is by observing the groups and communities the official accounts interact with. But Nimmo said this was all but impossible given that these fake accounts are designed to insert themselves into genuine communities and groups without being noticed.

Hidden messages

For the U.K. government, a deeper investigation by Facebook will likely offer greater insight into how big the Russian campaign was and potentially inform politicians about whether it had a material impact on the vote outcome. But, ultimately, the platform is just one of many channels Russia used to try to tip the scales toward Leave, analysts said.

The disinformation machine also encompassed Russia’s English-language media outlets like RT and Sputnik, other social networks like Twitter, private peer-to-peer messaging platforms like WhatsApp and Telegram, and even chain emails, according to one official at the East Stratcom Task Force, an EU initiative set up to identify and debunk Russian disinformation.

The official, who was not authorized to speak on the record, said these chain emails alone could reach “thousands or dozens of thousands of people” and warned of a similar reality on messaging apps.

Even if Facebook is able to dig up more information on the depth and scale of the Kremlin’s operation ahead of Brexit, the findings might only lead to more questions.