U.S. immigration policy is inextricably linked with national security and border control policies. But that wasnt always the case. The Sept. 11, 2001 terrorist attacks prompted a major shift in the way the country handles immigration, creating new government bodies and tightening restrictions on who is and isnt allowed in.

The 9/11 hijackers entered the country with a fact that immediately linked immigration with terrorism and national security, explained the Migration Policy Institute in a 2011 fact sheet .

Here are some of the key ways immigration has changed since 9/11: