“For far too long, our immigration system has been exploited by smugglers, traffickers and those with no legal right to be in the United States,” said Kirstjen Nielsen, the secretary of homeland security, on Thursday.

The Department of Homeland Security said that the claims for those returned would be adjudicated within a year, with an initial hearing held within 45 days. But immigration courts are already clogged with 800,0000 pending cases, raising questions about whether this timetable can be achieved, experts said.

The policy would apply both to some asylum seekers who try to enter the United States at border crossings and to those apprehended by the authorities after illegally touching United States soil.

Historically, undocumented single men have represented the bulk of those arrested and subsequently removed from the county. But since 2013, unaccompanied children and families have arrived in ever-larger numbers.

That influx of people was cited by a Department of Homeland Security fact sheet as justification for the policy. Children cannot be detained for more than 72 hours in border holding facilities, prompting the authorities to release their adult parents with them, and the fact sheet referred to a shift in the profile of immigrants reaching the border, “from a demographic who could be quickly removed when they had no legal right to stay to one that cannot be detained and timely removed.”

Analysts and lawyers raised other questions about the program.

Human rights groups questioned the ethics of sending people fleeing in search of safety back into Mexico, which itself is experiencing horrific violence. Politically, some have asked why Mexico’s new president would essentially agree to turn his country into a waiting room for American asylum seekers.

The Mexican government has cautioned that the details of who would be returned and when were still unclear. But in its statement, the government said that it would not accept unaccompanied children or people suffering from health problems. Mexico has yet to agree to accept families, but opened the door for future discussions.