U.N. Special Rapporteur Jorge Bustamante is conducting a fact-finding mission to examine the status of migrants' rights in the United States, but U.S. Immigration and Customs Enforcement prohibited him from making a scheduled stop at a family detention center in Taylor, Texas, the ACLU reported on Friday.

Immigrant families caught illegally crossing the southern border are housed at the Hutto Family Residential Facility while they await immigration proceedings. The detention center, which was formerly a medium-security prison according to the ACLU, is operated by the Corrections Corporation of America through a contract with the Department of Homeland Security.

Bustamante, who serves as special rapporteur for the human rights of migrants, has scheduled meetings with human rights and immigrants groups during his three-week trip to assess the treatment of illegal migrants in the United States. The tour of Hutto "was considered a major part of the Special Rapporteur's U.S. visit," the ACLU stated.

The ACLU currently represents 12 children from the Hutto facility in lawsuits against Homeland Security Secretary Michael Chertoff and several immigration officials. The lawsuits condemn the conditions of the facilities, faulting DHS officials in particular for providing only limited access to medical facilities and inadequate educational opportunities.

ICE spokesman Marc Raimondi said that Bustamante can visit the Hutto facility at the conclusion of the ACLU's case. "While we are under litigation, visits by non-governmental entities will not be granted," Raimondi said.

ICE offered to provide Bustamante with a tour of a different family detention facility in Pennsylvania, but the offer was rejected, Raimondi said.

According to the terms agreed to by U.N. member countries, an independent expert appointed by the U.N. Human Rights Council should have "access to all prisons, detention centers and places of interrogation."