President Barack Obama’s official response to the San Bernardino shooting, delivered in a Sunday evening address from the Oval Office, focused mostly on the question of how to defeat Islamic State in Iraq and the Levant (ISIL). But as part of his broader anti-ISIL strategy, the president called for some modest gun control measures.

Obama urged Congress to pass additional restrictions on the purchase of assault weapons and to “make sure no one on a no-fly list is able to buy a gun.” He framed the latter measure, in particular, as an act of common sense.

“What could possibly be the argument for allowing a terrorist suspect to buy a semiautomatic weapon?” said Obama. “This is a matter of national security."

Civil liberties activists beg to differ. The merits of keeping guns away from “terrorist suspects” aside, they argue that membership on the no-fly list is a poor measure of who should face such restrictions. They point to what they describe as an opaque and expansive process for including people on the list.

“There’s no constitutional bar to reasonable measures for regulating gun sales, and it’s possible that the no-fly list could serve as one tool in regulating gun sales,” said American Civil Liberties Union staff attorney Hugh Handeyside. “But it would have to be after major reform because of how error prone the no-fly list is and how overly broad the standards for placement on it are."

The Obama administration has long kept secret its decision-making process for placing suspects on the no-fly list. But last year the investigative news site The Intercept reported it had uncovered a 166-page secret document outlining the government’s criteria for adding people to its main terrorist suspect list, as well as the no-fly list.

The document, wrote Intercept reporters Jeremy Scahill and Ryan Devereaux, “reveals a confounding and convoluted system filled with exceptions to its own rules, and it relies on the elastic concept of ‘reasonable suspicion’ as a standard for determining whether someone is a possible threat."

“Because the government tracks ‘suspected terrorists’ as well as ‘known terrorists,’ individuals can be watchlisted if they are suspected of being a suspected terrorist, or if they are suspected of associating with people who are suspected of terrorism activity,” wrote Scahill and Devereaux.

In a follow-up piece, The Intercept reported that almost half the people on the government’s Terrorist Screening Database — which is then used to assemble the no-fly list — are listed as having "no recognized terrorist group affiliation."

The ACLU is currently challenging the constitutionality of the federal government’s no-fly list procedures in court, arguing that they violate the due process rights of those who find their names on the list. People on the no-fly list are currently permitted to challenge their inclusion, but ACLU attorneys say the appeal process does not pass constitutional muster, in part because it doesn’t provide complainants with adequate information regarding why they were placed on the list in the first place.

The Justice Department only revised its appeal process in April, following a June 2014 district court ruling that found that the old procedures didn’t satisfy due process requirements. ACLU maintains that the new procedures are still insufficient and that the government has therefore failed to comply with the 2014 ruling.

By the time Obama floated the idea on Sunday of barring people on the no-fly list from buying guns, Senate Republicans had already rejected a bill designed to implement the proposal in the aftermath of the San Bernardino shooting.

“Sometimes you’re only on that list because the FBI wants to talk to you about someone you know, not because you’re a suspect,” presidential candidate Sen. Marco Rubio, R-Fla., who objects to the proposal, told CNN on Sunday morning. “And, again, now you’re Second Amendment right is being impeded.”

A recent Government Accountability Office study found that 91 percent of attempted gun purchases by individuals on the FBI’s terrorist watchlist from 2004 to 2014 were successful.