Migrants who overstayed a visa are not eligible to be sent to Guantanamo Bay, the document says. But if the nature of a migrant's entry is unclear, the memo allows officials to assume that the person paid a criminal group to enter the U.S. and to send them to Guantanamo if they hail from a nation "where the preponderance of aliens from that country enter the United States in that fashion."

Instead, the agreement, signed on March 7 by top DHS and Pentagon officials, says the departments agreed to use the Guantanamo base to detain migrants with final deportation orders who have "a nexus to a transnational criminal organization (TCO) or criminal drug activity."

Officials defined "nexus" in broad terms. A nexus can be satisfied, the memo says, if migrants with final deportation orders are part of a transnational criminal group or if they paid one "to be smuggled into the United States." The latter condition could be used to describe many of the migrants and asylum-seekers who have illegally crossed the U.S. southern border, as criminal groups in Mexico largely control the illicit movement of people and drugs there.