Two memos released by the Department of Homeland Security on Tuesday lay out guidelines for the implementation of two executive orders President Donald Trump signed in January.
The memos detail the increased authority federal officials will have to detain and deport immigrants in the US illegally.
They also expand the pool of unauthorized immigrants subject to "expedited removal" - that is, removal that bypasses court proceedings - and describe the resources that will be provided to the Immigration and Customs Enforcement and Customs and Border protection agencies in order to fulfill the directives.
Also within the memos are provisionsthat seem likely to infuriate - and may well overwhelm - Mexican authorities.
Section H of the memo, titled "Implementing the President's Border Security and Immigration Enforcement Improvements Policies," details policies meant to streamline the deportation process for US authorities and to better use the resources required.
Provisions in the Immigration and Nationality Act authorize the Homeland Security Department "to return aliens arriving on land from a foreign territory contiguous to the United States, to the territory from which they arrived, pending a formal removal proceeding," the memo states.
"When aliens so apprehended do not pose a risk of a subsequent illegal entry or attempted illegal entry, returning them to the foreign contiguous territory from which they arrived, pending the outcome of removal proceedings saves the Department's detention and adjudication resources for other priority aliens," it adds.
As detailed by ProPublica, which reported the contents of the memo on Monday, these provisions would mean many immigrants not originally from Mexico - Guatemalans, Honduras, Salvadorans, or Haitians, among others - would still be put in the hands of Mexican authorities, who would have to decide their next steps.
Many of those migrants have previously been detained in the US, where they are allowed to apply for asylum. Under Trump's plan, they would have to do so from Mexico, communicating with US immigration officials by videoconference, presumably using facilities the Mexican government would have to provide.
"This would say if you want to make a claim for asylum or whatever we'll hear your case but you are going to wait in Mexico," a DHS official toldProPublica. "Those are details that are being worked out both within the department and between the US government and the government of Mexico … there are elements that still need to be worked out in detail."
Mike Vigil, a former chief of international operations for the US Drug Enforcement Administration who maintains contact with Mexican officials, said authorities there were aware the US intended to do something like this - and they were not keen on it.
Mexican officials are "of the opinion that Trump also wants to send individuals that were born Central American through Mexico, and they opposed that," he told Business Insider earlier this month.
"They want the United States to fly them or do whatever they have to do, but they don't want them going into or through Mexico," said Vigil, author of "Metal Coffins: The Blood Alliance Cartel."
The burden Trump - who has earned the scorn of much of Mexican society - apparently wants to foist on Mexico in the form of foreign deportees has attracted indignation from the government and public alike.
But the plan poses the additional risk of sapping Mexico's limited resources and recreating the very problems that inspired Trump's harsh crackdown.
"The addition of millions of unemployed people to economies that are already struggling would only multiply desperation and violence in Mexico and Central America, where security, justice, and social-service institutions are already overwhelmed," Adam Isacson, the senior associate for defense oversight at the Washington Office on Latin America, told Business Insider in January.
"There would probably be chaos," Isacson said. "And the desire to escape chaos would spur more migration northward."
The US has undertaken large-scale deportations of people to Mexico and Central America in the past. Such policies in the late 1990s and early 2000s sent many migrants back to places like Honduras and El Salvador - economically and politically fragile countries where they had no real roots. They soon became fodder for gangs like Barrio 18 and MS-13.
The surge of Central Americans coming to the US in recent years has been driven north by violence and instability - fostered in large part by those gangs - that is still present in the region. Their presence in Mexico has already taxed the resources of the government there.
Just as those deportees were vulnerable to criminal groups in northern Central America, waves of deportees returning to Mexico would be exposed to the criminal groups that have proliferated in that country, vulnerable to exploitation, extortion, or worse.
Despite the stricter border policies Trump has promised, many of those deported, no matter their country of origin, would likely look to return to the US.
"To the big wave of economic migrants would be more people fleeing violence, along with many thousands of husbands, wives, and parents who got separated from their loved ones by Trump's deportation raids," Isacson told Business Insider.
A Homeland Security official toldProPublica that the provisions cited the memo had been in place for years, and that the department was just pursuing its mandate.
But those policies, and the justification for them, are unlikely to sway Mexican authorities.
"I would expect Mexico to respond with an emphatic, 'No,'" Gustavo Mohar, a former senior Mexican immigration and national-security-policy official, told ProPublica.
