President Donald Trump addresses Congress last month.

The controversy around the communications between Donald Trump’s associates and Russian officials before he became president is highlighting something civil liberties advocates particularly dislike about the government’s foreign surveillance practices: when it spies on foreigners it believes to be overseas, which it can do without probable cause, it also sweeps up plenty of information pertaining to Americans.

The years-long debate in Washington over this sort of “incidental” collection of Americans’ communications data could get particularly intense this year because Section 702 of the Foreign Intelligence Surveillance Act (FISA) is set to expire at the end of 2017 if Congress does not reauthorize it. Installed in 2008, the law authorizes the National Security Agency to collect the electronic communications of “non-U.S. persons” who are “reasonably believed” to be outside the country. This surveillance inevitably collects data on Americans as well, for example if they happen to be communicating with a target.

Dan Coats, President Trump’s nominee for Director of National Intelligence, has called reauthorization a top priority. Civil liberties advocates, meanwhile, argue that Congress should pass reforms to install more checks and balances.

We don’t know how investigators ascertained the content of conversations between former National Security Advisor Michael Flynn and Russian ambassador Sergey Kislyak in December. It is possible, though, that they first learned of Flynn’s communications via surveillance of a Russian target authorized under FISA, perhaps Section 702. (The circumstances behind other reported surveillance involving Russian banks are even less clear.)

Section 702 authorizes the government to compel providers like Google and Facebook to disclose communications content associated with “targets” of foreign intelligence. It also gives it the power to make the companies that own and maintain the Internet’s backbone turn over communications content, including telephone calls. That includes not just communications sent to and from a target, but also communications that are about the target—even if the targeted person isn’t participating in the communication.

If investigators collect communications of an American that suggest that the individual is an agent of a foreign power or involved in criminal activity, they can try to obtain a separate court order or warrant for further surveillance. If the American’s information is not related to the foreign intelligence or thought to be evidence of a crime, the law calls for the government to “minimize” it and prevent it from being disseminated.

In practice, however, Section 702’s vague language has “allowed the government to craft rules that are permissive and contain multiple exceptions,” Elizabeth Goitein, co-director of the liberty and national security program at the Brennan Center for Justice at New York University School of Law, told the House Judiciary Committee last week. It “almost always” keeps Americans’ data for five years or more, and the NSA “routinely” shares raw data from Section 702 surveillance with the FBI and the CIA, Goitein told the committee.

What civil liberties advocates find most problematic, though, are so-called “backdoor” searches, wherein the NSA, CIA, or FBI uses identifiers associated with Americans to query data collected via Section 702 surveillance, even though that data was collected without a warrant. The FBI can even do this during criminal investigations that are not related to national security. The Foreign Intelligence Surveillance Court found in 2015 that such searches are in line with the Constitution, however.

Goitein and many others argue that the government’s refusal to make public a clear estimate of the number of Americans’ communications it collects via Section 702 surveillance, despite repeated requests from advocates and lawmakers including Senator Ron Wyden of Oregon, makes a fair debate over the law’s reauthorization impossible. Investigators collected more than 250 million communications under the law in 2011. Even if only a small fraction contained Americans’ data, that could still be a big number.

During Dan Coats’s Senate confirmation hearing last month, Wyden asked Coats if he would commit, if confirmed as director of National Intelligence, to getting a clear estimate of this number for Congress and the public before Congress must vote to reauthorize Section 702. Though he stopped short of guaranteeing the estimate, Coats told Wyden and other lawmakers that his team wants to “make sure you have all the information you feel you need.”