LAREDO, Texas (Reuters) - The United States is closing a legal loophole which has allowed tens of thousands of illegal immigrants to slip into the country and join the estimated 11 million undocumented foreigners already here. Under long-standing procedure along the U.S. border with Mexico, illegal crossers of nationalities other than Mexican -- dubbed OTMs by the Border Patrol -- have been entitled to a hearing before an immigration judge before they could be deported. Because of a severe shortage of space to hold them until the hearing, they were released after being fingerprinted and given a "notice to appear," a document stating they had agreed to show up at court at a certain date. The notice serves as a travel document allowing its holder past Border Patrol checkpoints on the roads leading from the border to the interior. Most OTMs do not show up for their hearing and meld into the population. Known as "catch and release," the practice has become part of an increasingly acrimonious debate over immigration policy and border security, an issue likely to loom large in Congress, next year's mid-term elections and the 2008 presidential poll. Homeland Security Secretary Michael Chertoff said last month his department's aim was to "return every single entrant -- no exceptions" but gave no deadline. Mexicans are usually returned immediately -- and most of them try again, some within hours. Along the border, agents say ending the practice will take time and depends on how quickly the government can build additional detention space told hold OTMs before they are sent to their home countries. OTMs were a relatively minor problem until 2003, when word of the loophole spread and triggered a growing flood -- including thousands of Brazilians inspired by a popular soap opera, "America," whose sultry star plays an illegal immigrant who swam the Rio Grande and made good in the United States.