Early Wednesday morning, President Trump appeared to try casting partial blame for Tuesday’s terrorist attack in New York on Minority Leader Chuck Schumer.

Trump tweeted: “The terrorist came into our country through what is called the ‘Diversity Visa Lottery Program,’ a Chuck Schumer beauty. I want merit based.’”

That detail hasn’t been officially confirmed. ABC7 News in New York reported that the man who killed eight with a truck and cited ISIS as his inspiration immigrated from Uzbekistan through the program, which is also known as the Green Card Lottery, although other outlets have not yet confirmed his use of the Diversity Visa Lottery.

But Trump and the conservative media quickly blamed the program and Schumer, who introduced it in 1990, for allowing a dangerous man into the country. Up to this point, however, little has been said about this specific program in the public debate over immigration. What is this program?

The Diversity Visa Lottery Program exists to allow people from countries with relatively few immigrants into the United States: People from countries with more than 50,000 immigrants to the U.S. in the past five years are not eligible. The State Department offers 50,000 of these visas a year to diversity immigrants, as they are called, and recently most have gone to Africans.

To qualify for the lottery, which is done by a randomized computer drawing, people must have a high school education or two years in a job that requires training. Listeners of This American Life might remember the story of Abdi, a Somali refugee living in Kenya, who won his “golden ticket” through the program.

Then-Rep. Schumer’s proposed program was passed with bipartisan support and went into effect in 1995 as part of the Immigration Act of 1990. But the concept of a visa lottery dates back further, to the mid-1980s, when, according to CUNY Brooklyn College political scientist Anna Law, who tweeted about the program Wednesday morning, prominent Italian American and Irish American members of Congress “argued that the ‘old-seed’ immigrant groups were being shut out by newer groups from Asia and Latin America.”

At the time, the number of immigrants from Asia and Latin America had been rising as the number of Western European immigrants declined—a result of the 1965 immigration act, which eliminated quotas and prioritized reuniting families and bringing in skilled laborers.

To combat the trend, Congress in the 1980s amended the immigration policy to allow for a number of first-come, first-served visas, a major portion of which were doled out to the Irish, according to the Washington Post. The 1990 act that made the current visa lottery was championed by Democratic Rep. Brian Donnelly, the same congressman who had proposed the 1980s amendment. In the early ’90s, according to the Post, those visas still went in large numbers to the Irish, until the Irish economy improved later in the decade. The bill, Law wrote in a 2002 paper in the Journal of American Ethnic History, was “a policy to benefit their ethnic constituents in the time honored practice of pork barrel politics.”

Over time, the program did actually become diverse. It now largely serves Africa and Eastern Europe. And in recent years, some conservative lawmakers started arguing the program presented a national security concern. Earlier this year, Trump endorsed a bill introduced by Republican Sens. Tom Cotton and David Perdue that planned to dramatically reduce the number of green cards awarded with a more “merit-based” system. The RAISE Act would completely eliminate the program but is seen as unlikely to pass.

According to the Washington Post, there are no recent conclusive data on any potential national security risks from the program. A 2007 report from the Government Accountability Office looking into the program warned it was “vulnerable to fraud,” a warning the Bush administration State Department ignored because it considered its screening to be adequate. A 2011 Congressional Research Service report mentioned just one violent case, a shooting by a diversity immigrant’s spouse in Los Angeles. Neither report found there to be a real threat of terrorism.