But under the new rule, set to be announced in the next 48 hours, Border Patrol agents would immediately return anyone to Mexico — without any detainment and without any due process — who attempts to cross the southwestern border between the legal ports of entry. The person would not be held for any length of time in an American facility.

Although they advised that details could change before the announcement, administration officials said the measure was needed to avert what they fear could be a systemwide outbreak of the coronavirus inside detention facilities along the border. Such an outbreak could spread quickly through the immigrant population and could infect large numbers of Border Patrol agents, leaving the southwestern border defenses weakened, the officials argued.