They had visas? Who knew.... susie
"IRS issues ITINs to foreign nationals and others who have federal tax reporting or filing requirements and do not qualify for SSNs. A non-resident alien individual not eligible for an SSN, who is required to file a U.S. tax return only to claim a refund of tax under the provisions of a U.S. tax treaty, needs an ITIN.
Examples of individuals who need ITINs include:
Non-resident alien filing a U.S. tax return and not eligible for an SSN
U.S. resident alien (based on days present in the United States) filing a U.S. tax return and not eligible for an SSN
Dependent or spouse of a U.S. citizen/resident alien
Dependent or spouse of a non-resident alien visa holder"
http://www.irs.gov/individuals/article/0,,id=96287,00.html#who
Some did have visas.
It's cheaper to get a visitor's visa and a plane ticket from Mexico city to Houston, than paying a smuggler to bring you in.
Since 9/11, it's become more difficult to obtain the visas, and more expensive to have the smuggler bring you in.
Some of the illegal aliens in the country today, came in with visitor's visas years ago.
All the 9/11 terrorists entered the country legally, and overstayed their visas.