It is a minor point, but they are not ipso facto criminals. To be a criminal you have to violate the criminal code -i.e. rape, murder, armed robbery, that sort of thing. You are not a criminal if you get a speeding ticket, or, if you build a fence to close to the property line and without a city permit - no matter how unlawful these things otherwise are.
"There are an estimated 80,000 criminal illegal aliens in the country, which are people who have committed crimes in the United States and are due for deportation but remain at large. An estimated 4,000 of those criminal illegal aliens are from countries that support terrorism."
Source: Lou Dobbs
http://www.cnn.com/2004/US/12/13/immigration.bill/index.html
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Time magazine quote:
“The number of illegal aliens flooding into the US this year will total 3 million - enough to fill 22,000 Boeing 727-700 airliners, or 60 flights every day for a year.”
http://www.immigrationshumancost.org/text/crimevictims.html