Since the election, Donald Trump’s team has argued he never advocated for a Muslim registry nor will he, but Trump himself has yet to reject the proposal outright. Photograph: Win McNamee/Getty Images

Throughout his presidential campaign, Donald Trump made clear his intent to pursue aggressive tactics against Muslims as part of his approach to national security.

The most prominent was his proposal to ban all Muslim immigration to the US, which remains on Trump’s website even today. But this week, a key question before Trump’s pending administration was whether it would implement a Muslim registry.

Concerns this week over a possible database of Muslims focused on a discussion between Trump and Kris Kobach, the Kansas secretary of state, for a potential cabinet position. Kobach was photographed on his way into the meeting with Trump on Monday carrying documents outlining plans requiring “special registration” of immigrants from certain “high-risk” countries, a method used in the past to create a database of mostly Muslims.

Trump’s incoming chief of staff, Reince Priebus, also declined to explicitly rule out such a registry, even as he said there would be no database based on religion.

The lack of clarity has prompted many to wonder: is there any precedent for such a registry, and would it even work?

What has Trump said?

As a candidate, Trump did not specifically campaign on the need for a Muslim registry. But when Trump was asked, on multiple occasions, if he favored a broader Muslim registry, he both declined to rule it out while signaling potential support for the idea.

Trump also called for a database for Syrian refugees coming to the US, and on 20 November was widely perceived as embracing a broader Muslim registry in the wake of the terrorist attacks in Paris.

When an MSNBC reporter asked if “there should be a database or system that tracks Muslims in this country”, Trump responded: “There should be a lot of systems. Beyond databases. I mean, we should have a lot of systems.”

Pressed on whether his administration would implement a Muslim registry, Trump proceeded to say he “absolutely” would. Politifact suggests he may have been referring at this point to the border wall, as Trump had deviated to discussing his plan for the southern border when the reporter followed up.

But he also said “it would just be good management” when the reporter asked again about creating a database of Muslims.

Since his victory in the presidential race, Trump’s campaign has argued he never advocated for a Muslim registry nor will he. But Trump himself has yet to put an end to the discussion by rejecting the proposal outright, and his meeting with Kobach only further fanned the flames.

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Donald Trump: we need to track all Muslims in America – archive video

Has there ever been a Muslim registry?

The precedent cited by both proponents and opponents of a Muslim registry is the National Security Entry-Exit Registration System, known as NSEERS, established after the September 11 attacks under President George W Bush. Kobach, who served in the Department of Justice at the time, was incidentally the program’s chief architect.

NSEERS was framed as an effort to focus not on religion, but on country of origin. But of the 25 countries on its list requiring “special registration”, 24 were Muslim-majority (the other was North Korea), and Muslims and Arabs were disproportionately targeted.

Men from those countries over the age of 16 had to register upon entry to the US, as did those already in the country, at the now defunct Immigration and Naturalization Service. The process included fingerprinting, photographs and interrogation, as well as a designation to check in at regular intervals with immigration authorities.

More than 100,000 individuals were registered before the program’s suspension in 2011, and at least 14,000 were deported – mostly for overstaying visas. Not a single terrorism charge was publicly reported from the database, which even Bush administration officials later conceded was not simply a failure in terms of intelligence gathering but also counterproductive for marginalizing Muslim communities at home and abroad.

“To my knowledge, not one actual terrorist was identified,” James W Ziglar, a former INS commissioner, told the New York Times in 2004.

“But what we did get was a lot of bad publicity, litigation and disruption in our relationships with immigrant communities and countries that we needed help from in the war on terror.”

How would a Trump administration register Muslims?

Trump has not called for blanket registration of all Muslims in the US or those seeking to immigrate from other countries. Doing so would not only be unconstitutional, but also impossible to implement.

There is not a precise figure for how many Muslims currently live in the US, as the Census Bureau does not collect data based on religion. Estimates have ranged from 3-7 million Muslims in the US, but in any case they account for less than 1% of the country’s adult population.

As of 2010, Muslims made up just under a quarter (23.2%) of the global population, with roughly 1.6 billion around the world, according to the Pew Research Center. Two of the three countries with the largest Muslim populations – Indonesia and India – are close US allies, as is Turkey, which has the eighth-largest Muslim population in the world.

Trump’s campaign, both when discussing his proposed Muslim ban and a database of any kind, has suggested he would focus on countries perceived as “high-risk”, following the phrasing of the Bush administration when setting up NSEERS.

The Muslim-majority countries in that program were: Afghanistan, Algeria, Bahrain, Bangladesh, Egypt, Eritrea, Indonesia, Iran, Iraq, Jordan, Kuwait, Lebanon, Libya, Morocco, Oman, Pakistan, Qatar, Saudi Arabia, Somalia, Sudan, Syria, Tunisia, the United Arab Emirates and Yemen.

Would there be a legal challenge?

It is unconstitutional to discriminate on the basis of religion, and civil rights groups have already threatened to sue if a Trump administration proceeds with anything even resembling a Muslim registry.

Omar Jadwat, a senior staff attorney at the ACLU’s Immigrants’ Rights Project, told the Guardian the question before a court or in a legal challenge would be what a database was intended to do.

“Trump himself and other people on his team have been clear about their intent to single out or discriminate against Muslims in this way,” he said.

“I don’t think they can un-ring that bell and try to pretend there’s some other neutral reason for what they’re doing, especially against the backdrop of NSEERS itself having been a total failure.”

Jadwat added that the ACLU would sue “if [the Trump administration] created a Muslim registry, whether it be by name or by even without naming it as such”.

New York City would also sue the federal government if Trump required Muslims to register in a database, Mayor Bill De Blasio said this week in a speech that denounced several of Trump’s proposals. “We will use all the tools at our disposal to stand up for our people,” he said.