But since the 2000 census was taken, much has changed for many of the roughly 8.7 million undocumented immigrants living in America. Some lost their jobs and returned to their native country.
And after the Sept. 11 attacks, the focus has shifted even more to closing immigration loopholes and tightening border patrols amid greater suspicion of foreigners.
For instance, the Justice Department has targeted for deportation thousands of men known to be from countries where Osama bin Laden's al-Qaida network operates.
The men, thought to be in the country illegally, have ignored orders to leave, Justice officials have said. Many, but not all, of those targeted are of Middle Eastern descent.
Preliminary estimates from the 2000 census show that about 115,000 people from Middle Eastern countries live in the United States illegally, or in "quasi-legal" status - refugees or people seeking political asylum.
While the vast majority of Middle Eastern immigrants are not terrorists, "the fact that tens of thousands of people from that region and millions more from the rest of the world can settle in the United States illegally means that terrorists who wish to (enter the United states) face few obstacles," said Steven Camarota, research director at the Center for Immigration Studies.
"We can't protect ourselves from terrorism without dealing with illegal immigration," Camarota said Tuesday. His group's mission statement says that it seeks "fewer immigrants but a warmer welcome for those admitted."
Critics favoring more restrictive immigration policy are unfairly using the terrorist attacks to advance their agenda, said Cecilia Munoz, vice president for policy at the National Council of La Raza, a Hispanic advocacy group.
"It is unreasonable to use this as an opportunity to stir up fear and division," said Munoz.
Census estimates show that the number of undocumented immigrants in the United States rose nearly 5 million during the decade, to 8.7 million in 2000. Of that total, almost 3.9 million, or 44 percent, were from Mexico.
The Census Bureau did not ask about citizenship status on its forms. Estimates were derived from Census 2000 data and Immigration and Naturalization Service records.
Census Bureau analyst Joe Costanzo stressed the figures were preliminary and could change as more research becomes available.
But for many demographers, the estimates provide valuable insight into how the U.S. population changed during the 1990s.
"I wouldn't necessarily raise a red flag over 100,000 people from the Middle East," said Jeffrey Passel, a demographer with the Urban Institute, a liberal-leaning think tank. "However, 8.7 million is a very big number - and it does point to an overall problem in controlling immigration."
Pro-immigrant groups like the National Immigration Forum favor stricter measures in the way student and tourist visas are distributed as a key step in tightening security.
The Hispanic population, specifically, grew faster than expected during the decade. Latinos numbered 35.3 million in the 2000 census, rivaling blacks as the nation's largest minority group.
The number of undocumented men from Mexico grew from 396,000 in 1990 to 2.1 million in 2000. Many of them settled in Northeast and Midwest urban centers whose white residents have fled to the suburbs, said University of Michigan demographer William Frey.
"Now, the population totals of many cities in the Midwest and Northeast may be left vulnerable by these stricter immigration laws."
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