House and Senate investigators have grown increasingly concerned that Facebook is withholding key information that could illuminate the shape and extent of a Russian propaganda campaign aimed at tilting the U.S. presidential election, according to people familiar with the probe.

Among the information Capitol Hill investigators are seeking is the full internal draft report from an inquiry the company conducted this spring into Russian election meddling but did not release at the time, said these people who, like others interviewed for this story, spoke on the condition of anonymity to discuss matters under investigation.

A 13-page “white paper” that Facebook published in April drew from this fuller internal report but left out critical details about how the Russian operation worked and how Facebook discovered it, according to people briefed on its contents.

Investigators believe the company has not fully examined all potential ways that Russians could have manipulated Facebook’s sprawling social media platform.

A particularly sore point among Hill investigators is that Facebook has shared more extensive information — including ads bought through fake Russian accounts — with special counsel Robert S. Mueller III, who is conducting a separate probe into alleged coordination between Russia and President Trump’s campaign.

Some members of the House and Senate intelligence committees were irritated that Facebook staff showed them copies of the ads but would not let the committees keep the documents for further study.

“It’s always a little problematic when you come before a committee and show them documents and then take them back,” said Sen. Mark R. Warner (Va.), the ranking Democrat on the Senate Intelligence Committee. “My hope is they will be more cooperative going forward.”

Facebook spokesman Tom Reynolds said the company has worked to be as transparent as possible.

“We have voluntarily and proactively briefed both members and committee staff and look forward to continued cooperation,” he said. “Federal law and the ongoing investigation may limit what we can release publicly.”

The investigators’ frustrations follow Facebook’s announcement earlier this month that accounts traced to a shadowy Russian Internet company had purchased at least $100,000 in ads during the 2016 election season.

Warner and his Democratic counterpart on the House Intelligence Committee, Rep. Adam B. Schiff of California, have been increasingly vocal in recent days about their frustrations with Facebook.

Congressional investigators are questioning whether the Facebook review that yielded those findings was sufficiently thorough.

They said some of the ad purchases that Facebook has unearthed so far had obvious Russian fingerprints, including Russian addresses and payments made in rubles, the Russian currency.

Investigators are pushing Facebook to use its powerful data-crunching ability to track relationships among accounts and ad purchases that may not be as obvious, with the goal of potentially detecting subtle patterns of behavior and content shared by several Facebook users or advertisers.

Such connections — if they exist and can be discovered — might make clear the nature and reach of the Russian propaganda campaign and whether there was collusion between foreign and domestic political actors. Investigators also are pushing for fuller answers from Google and Twitter, both of which may have been targets of Russian propaganda efforts during the 2016 campaign, according to several independent researchers and Hill investigators.

“The internal analysis Facebook has done [on Russian ads] has been very helpful, but we need to know if it’s complete,” Schiff said. “I don’t think Facebook fully knows the answer yet.”

Google spokeswoman Andrea Faville said the company is “always monitoring for abuse or violations of our policies and we’ve seen no evidence this type of ad campaign was run on our platforms.” A Twitter spokesman declined to comment. Warner said Twitter plans to brief lawmakers in the coming weeks.

Trump and campaign officials have denied any coordination with Russia during the election. Russian President Vladmir Putin also has denied intervening to help get Trump elected.

Facebook began examining the ads following a May visit to Silicon Valley by Warner, who at the time asked executives if they had examined whether Russians used the company’s advertising system, according to people briefed on the discussions.

The delay in probing the possibility that Russians had used Facebook’s multibillion-dollar advertising system in its propaganda campaign has frustrated outside experts, who say the company has been slow to recognize the seriousness of the issues.

“All I can say is, ‘Wow,’ ” said Zeynep Tufekci, an associate professor at the University of North Carolina at Chapel Hill who studies social media companies’ impact on society and governments. “Given the scale of the misinformation campaign, it’s pretty obvious that ads would be a vector. They are an ad company.”

Warner said the company still has not yet gone far enough, noting that Facebook shut down 50,000 accounts and pages in France ahead of the July election of President Emmanuel Macron because of concerns that they were fake and violated Facebook policy. So far Facebook has reported shutting down 470 that it traced to ad purchases during the U.S. election cycle.

“When I was raising this issue, they were kind of dismissive,” Warner said. “They took down 50,000 accounts in France. I find it hard to believe they’ve only been able to identify 470 accounts in America.”

Warner said his committee has asked Facebook new questions that he hopes prompt the company to embark on a deeper investigation. Congressional investigators last week asked Facebook, for example, to investigate whether other “troll farms” identified in Belarus, Macedonia and Estonia also used Facebook pages and ads, congressional staffers said.

When Facebook began studying its political ads in May, questions about the use of the social-media platform as a propaganda tool had been circulating for many months. Days after the November election, chief executive Mark Zuckerberg called the notion that manipulation of Facebook had influenced the election a “crazy idea.”

The company’s report in April didn’t mention Russia directly saying, “Facebook is not in a position to make definitive attribution to the actors sponsoring this activity.”

But the company seemed to suggest that it knew more information, noting that its data “did not contradict” assertions from intelligence agencies in January that Russia engaged in a vast campaign to manipulate the U.S. election and used its digital arsenal to do so.

In the white paper, Facebook noted new techniques the company had adopted to trace propaganda and disinformation.

Facebook said it was using a data-mining technique known as machine learning to detect patterns of suspicious behavior. The company said its systems could detect “repeated posting of the same content” or huge spikes in the volume of content created as signals of attempts to manipulate the platform.

As recently as July 20, a Facebook spokesman told CNN, “We have seen no evidence that Russian actors bought ads on Facebook in connection with the election.”

A Facebook official said Monday that the statement was “accurate at the time we shared it,” noting that the Russian ads were discovered in the more recent review.

Under federal law, it is illegal for a foreign national or corporation to make a contribution or expenditure "in connection with a Federal, State, or local election."

Facebook officials have said that most of the ads made no explicit reference in favor of Trump or Democratic opponent Hillary Clinton. Campaign finance experts said it is impossible to know whether the ads paid for by a Russian company broke the law without analyzing the content of the ads themselves.

If the ads were overtly political — that is to say, they advocated the election or defeat of a specific candidate — then they would violate the prohibition on foreign national spending, legal experts said.

However, Russian-financed ads could have still run afoul of election law if they were placed on Facebook or targeted at certain voters in coordination with a campaign — one of the central questions of the ongoing Russia probes. In that scenario, the ads would not have to explicitly advocate for a candidate to be illegal.

Dwoskin reported from San Francisco. Matea Gold and Tom Hamburger contributed to this report.