Report shows that the attack of 9/11 was not an isolated instance.

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Study: Terrorists exploit immigration laws

WASHINGTON (AP) -- Some used false documents to enter the United States; others let their legal visas expire once in the country. And at least 21 foreign nationals became naturalized U.S. citizens before being charged or convicted as terrorists.
In all, at least 94 foreign-born visitors accused of terror activity between 1993 and 2004 exploited federal immigration laws to enter or remain in the United States, according to a study being released Tuesday.
Distributed by the Center for Immigration Studies, an advocate for stricter immigration policies, the report provides newly compiled data on U.S. terror arrests to illustrate gaps in the nation's border security, visa approval and immigration systems. It was written by Janice Kephart, who served as counsel to the 9/11 Commission that investigated missteps leading to the Sept. 11, 2001, attacks.
"The attack of 9/11 was not an isolated instance of al-Qaida infiltration into the United States," the 46-page report found. "
In fact, dozens of operatives both before and after 9/11 - other than the 9/11 hijackers - have managed to enter and embed themselves in the United States, actively carrying out plans to commit terrorist acts against U.S. interests or support designated foreign terrorist organizations," the report concluded. "For each to do so, they needed the guise of legal immigration status to support them."
Tightening U.S. borders has become a top priority for Homeland Security Secretary Michael Chertoff, who last week called immigration enforcement an issue of utmost importance….Since June 2003, the department has investigated 7,100 cases out of millions of immigrants suspected of violating temporary visas, resulting in 1,339 arrests, Knocke said.

The article also comments that in recent studies indicate immigration related cases made up for nearly 33% of all federal prosecutions last year, more than any other crime.