Editor’s Note: A version of this story first published in 2019.

CNN
 — 

It’s a surprising fact that’s often overlooked in the immigration debate.

Undocumented immigrants pay billions of dollars in federal taxes annually, between tax returns filed and taxes deducted from paychecks, experts estimate.

Here’s a look at why – and how – that’s happening:

The National Immigration Law Center breaks down a number of reasons why undocumented immigrants pay taxes, including:

• It shows they’re complying with federal tax laws.

• It can help them demonstrate “good moral character” if they later have an opportunity to legalize their immigration status.

• Tax return records could be used to document work history and presence in the US, steps that may help them be eligible for legal immigration status in the future if lawmakers pass immigration reform.

Critics of illegal immigration have long argued that undocumented immigrants who pay taxes are able to do so because they’re using stolen Social Security numbers. But millions of federal tax dollars are paid every year by people who don’t have Social Security numbers at all.

Instead, they file using what’s known as an Individual Taxpayer Identification Number (ITIN).

The Bipartisan Policy Center, a Washington-based think tank, notes that “most experts believe the vast majority of tax returns filed with ITINs today are filed by undocumented immigrants.”

Some noncitizens who legally immigrated to the United States also pay taxes using this method.

In 2015, according to the IRS, 4.35 million tax returns were filed using ITINs, accounting for more than $13.6 billion in taxes.

In addition to tax return filings, officials have estimated that undocumented immigrants also contribute billions to Social Security annually through payroll tax deductions. In 2010, for example, the Social Security Administration estimated that payments from unauthorized workers accounted for about $12 billion in tax revenue for Social Security.

In recent years, immigrant rights advocates’ posts on Facebook, Twitter and TikTok during tax season have drawn attention to the issue.

“Undocumented immigrants pay billions in taxes to fund programs they can’t access,” the National immigration Law Center wrote in a series of recent social media posts.

In 2017, Belén Sisa’s post about her experience paying taxes went viral.

“Wanna tell me again how I should be deported, contribute nothing and only leech off this country while the 1% wealthiest people in this country steal from you everyday?” wrote Sisa, who was a college student in Arizona at the time.

The beneficiary of the Deferred Action for Childhood Arrivals program later told CNN she felt it was important to speak out.

“I wanted to show people that we’re here, and that we come from all over the world, and that we contribute more than people think that we do,” she said.

Jose Antonio Vargas, the Pulitzer Prize-winning journalist-turned-activist who’s turned his struggles as an undocumented immigrant into a platform for advocacy, tweeted that he was finalizing his taxes over the weekend.

“Yes,” he wrote, “undocumented immigrants are helping fund the very system that detain and deport us.”