The omission of online ads could be a potential hurdle for those investigating alleged Russian meddling in last year’s U.S. presidential election, according to the campaign finance lawyers, who are not involved in the probes.

Since 1974, the United States has barred foreign nationals from giving money to campaigns and it later barred them from donating to political parties. The laws also prohibit foreign nationals from coordinating with a campaign and from buying an ad that explicitly calls for the election or defeat of a candidate.

At issue are laws concerning a category of ads known as “electioneering communications” that address where a candidate stands on an issue, that spread news stories about a candidate or that attack a candidate without directly calling for his or her defeat.

Foreign nationals cannot spend money on electioneering communications but the term under U.S. law applies only to communications made by broadcast, cable or satellite—with no mention of the internet.

A related law does bar foreign nationals from expressly advocating the election or defeat of a candidate in any advertising medium. But as long as online ads do not call on people to vote for a specific candidate, “they are not prohibited as a campaign finance matter,” said Jan Baran, a Washington lawyer who frequently represents Republican candidates.

U.S. Justice Department special counsel Robert Mueller was appointed in May to investigate alleged Russian interference as well as potential collusion between Moscow and associates of President Donald Trump. The Russian government says it did not meddle and Trump has denied collusion.

Facebook, which has become a leading platform for online political ads, has said that it has not found any evidence that Russian agents were buying ads.

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Facebook said in April, however, that it had become a battleground for governments seeking to manipulate public opinion in other countries and outlined new measures to combat what it called “information operations.”

Trump’s election campaign spent some $70 million on highly targeted Facebook advertising last year, Brad Parscale, the digital director of the campaign, told Reuters this year.

The absence of rules governing online advertising ties the hands of the U.S. Federal Election Commission, said Ann Ravel, a former Democratic appointee to the commission.

“The law seems to totally exempt it and therefore it is not possible either to reasonably determine who is behind those ads, do an investigation to find out or penalize for that activity,” Ravel said.

Other laws could come into play in an investigation. The U.S. Foreign Agents Registration Act restricts publicity paid for by a foreign government such as Russia.