Federal appeals judges in Washington will soon decide whether the public has the right to see secret Justice Department documents setting out the legality of surveillance practices – which powerful senators say amount to a body of secret law.

A panel of three judges from the District of Columbia circuit court of appeals met on Tuesday morning to hear arguments related to the government's ability to withhold from public view a 2010 ruling from the Justice Department's Office of Legal Counsel (OLC) about the FBI's ability to get customer data from telecommunications firms without any legal encumbrances.

The case, Electronic Frontier Foundation v Department of Justice, is one of several recent transparency lawsuits launched in the wake of the Snowden revelations in the Guardian and other news organisations. The lawsuits are designed to shed light on what senators Ron Wyden and Mark Udall, members of the intelligence committee, call anti-democratic "secret law." But the surveillance practice at issue is not believed to involve the sort of bulk data collection that the NSA engages in.

It is believed to be the first time the DC appeals court has ever heard a case applying the Freedom of Information Act to the OLC, a Justice Department body that assesses the legality of administration policies – a remarkable rarity considering that Washington is the venue for many national-security cases under the Freedom of Information Act.

Directly at issue during the 45-minute session on Tuesday was whether the OLC’s opinions are adopted by the agencies they advise – in this case, the FBI and possibly the CIA – or whether the office merely passed along non-operational guidance.

It is unlikely the judges would compel the Justice Department to disclose the OLC opinion should they decide it was not central to and impactful of the FBI surveillance practice, which is described obliquely in a heavily redacted Justice Department inspector general’s report from January 2010.

The practice is apparently no longer used by the FBI. Electronic Frontier Foundation lawyer Mark Rumold called it a “devolution” of a form of administrative subpoena issued by FBI for customer records.

Between 2003 and 2006, the inspector general report indicates, the FBI kept telecommunications personnel “embedded in the FBI itself”. Requests for customer data – previously subject to court-issued subpoenas or warrants – amounted to “Post-It notes,” Rumold said.

At least one judge on the panel indicated impatience with Rumold’s argument that the OLC perspective was “controlling” on federal agencies like the FBI.

“That doesn’t make a lot of sense,” said Judge David Sentelle.

Daniel Tenny, a Justice Department lawyer, contended that the OLC opinion amounted to private guidance from administration lawyers, typically exempt from transparency laws.

But Tenny conceded that it was unlikely that a federal agency could disregard a formal OLC opinion about the legality of an action performed by the agency, or a power it claimed to possess.

“As a matter of executive practice, we would expect that they wouldn’t,” Tenny said.

Judge Harry Edwards, Sentelle’s colleague on the panel, seemed to doubt Tenny’s contentions that the Office of Legal Counsel opinion did not have a controlling effect on the FBI and that the public did not have an interest in seeing the opinion.

“You’re blowing it off too quickly,” Edwards said.

The third judge on the panel, Sri Srinivasan, questioned both attorneys and gave little impression as to his inclinations. The panel is likely to take months before issuing a ruling.

After the hearing, Rumold said it was possible that the CIA may rely on the OLC opinion for what the New York Times recently described as a practice of collecting financial data in bulk. He also warned that the FBI’s claim not to perform the unencumbered data collection from telecommunications firms was a reversible policy choice now that the OLC had blessed it.

“It’s the same problem in the NSA context,” Rumold said. “The blossoming of secret surveillance law under government privilege.”