In addition to the challenge posed to the United States by Colombian drug trafficking, illegal Colombian immigrants in the United States are an issue in Colombia-U.S. relations. According to figures from the U.S. Department of Homeland Security, Colombia is the fourth-leading source country of illegal immigration to the United States. According to its estimates, the number of illegal Colombian residents in the United States almost tripled from 51,000 in 1990 to 141,000 in 2000. According to the US Census Bureau, the number of authorized Colombian immigrants in the United States in 2006 was 801,363.