As many as 4 million people — many of them Muslims — may lose their Indian citizenship on Saturday, accused of being “infiltrators” by the government of Prime Minister Narendra Modi.

Modi’s ruling Bharatiya Janata Party (BJP) is preparing to release the final version of a controversial list called the National Register of Citizens, the result of a six-year effort to ostensibly catalog all the legal residents of the northeastern state of Assam.

But the list has been mired in controversy. The last time a draft was released, in 2018, 4.1 million people were left off, putting them at risk of being made stateless. Many of them were poor, illiterate and/or members of the Muslim minority.

Observers are worried that the final list, published on Saturday, will result in families being split up, incarcerated, and even being chased out to Bangladesh — a country many have never set foot in. And rights groups are worried that the list could set off a chain of events similar to the Rohingya crisis in Myanmar in 2017, when roughly 750,000 members of the Muslim Rohingya minority were persecuted, stripped of their Burmese citizenship and forced to flee to Bangladesh.

Others fear that this is the start of a disturbing trend. Modi’s Home Minister, Amit Shah, has promised to expel “infiltrators” from the rest of India, using Assam as a testing ground. “We will implement National Register of Citizens in the entire country,” Shah said when the BJP returned to power for another five years in May. “We will remove every single infiltrator.”

Here’s what to know about India’s National Register of Citizens.

Why does the Indian government want a National Register of Citizens?

The idea has been floated before, partly because of the local history of Assam, where indigenous Assamese people have long held anxieties about being displaced by waves of Bengali immigration — first during the partition of British India in 1947, then on the eve of the India-Pakistan war in 1971, and still today over the state’s porous border with Bangladesh.

In 2013, the Indian Supreme Court ruled that the NRC be updated. The list was first drawn up in the 1950s but fell into disuse.

But the ruling BJP, which came to power the following year, has seized on the issue, because it dovetails conveniently with the party’s wider Hindu nationalist message: that India’s Hindus are being displaced by Muslims. Shah, the Home Minister, has previously referred to illegal immigrants as “cockroaches” — widely seen as a dog-whistle for Muslims. (Muslims make up some 14% of India’s population.)

“BJP leaders say Muslims have more children and are altering the demographic makeup of India,” says Meenakshi Ganguly, South Asia director at Human Rights Watch. “That kind of language is very concerning.”

“The BJP has a very Trump-like way of speaking about illegal immigrants,” she adds.

Can’t everyone in Assam simply prove they are Indian citizens?

It’s not that easy. Activists say the Indian state has put an unfairly high burden of proof on residents to prove they have the right to remain — requiring documentation dating back to March 24 1971, the day before war with East Pakistan (today known as Bangladesh) erupted. For families who don’t keep meticulous records, who have faced persecution, or who are unable to navigate India’s complex bureaucracy, that can be difficult.

Even when all the papers are found, some families have still experienced obstacles. In 2018, TIME reported on a Muslim family whose members were left off the draft version of the NRC despite providing a stack of documents going back 80 years. If their names are not included on the final version released Saturday, they face being thrown into detention centers.

“We have requested the government to make sure the process is not arbitrary, not discriminatory,” says Ganguly of Human Rights Watch. But only when the NRC is published will the full repercussions become clear. If anywhere close to the 4.1 million people left off the list last time are still not included, rights groups fear a humanitarian crisis.

“We are very worried about something like the Rohingya crisis playing out again,” Ganguly tells TIME. “There is a similar anti-Muslim public discourse [to that in Myanmar] that is underlying all of this.”

Will families be separated?

That has already happened. In 2011, a new mother called Sahida Bibi was deemed a foreigner by a tribunal, and thrown into a detention camp with her newborn twin sons. One of her twins died two weeks into their detention from a respiratory illness. “He made a strange noise and as I picked him up he was gone,” she told news site Scroll.in.

Separated from her husband and still grieving, Bibi was forced to remain in the detention camp for 10 months more before she was freed. A court deemed her an Indian citizen, and said she should never have been detained in the first place.

Can India really deport millions of people to Bangladesh?

It’s not likely. Bangladesh has called the situation in Assam an “internal issue” for India to deal with. And Bangladesh is already struggling to accommodate the roughly 1 million Rohingya refugees who fled state-sanctioned murder, rape and violence in Buddhist-majority Myanmar.

And, since most of the people left off the NRC don’t hold Bangladeshi citizenship, or any other apart from Indian, it would be illegal under international humanitarian law for India to strip their Indian citizenship, thus making them stateless.

So it’s more likely that India will have to send people to detention camps inside its own territory. Ten new detention camps are under construction in Assam, and police have been mobilized in the state ahead of the publication of the NRC.

“Even if there are people who are Muslims who have been living in Assam since the 1970s, that’s three generations of people,” says Ganguly. “So how do you even decide where they are going to go?”

Write to Billy Perrigo at billy.perrigo@time.com.