A day later, Sessions rescinded an Obama administration plan to phase out Bureau of Prisons contracts with private prisons, saying it "changes long-standing policy and practice, and impaired the Bureau's ability to meet the future needs of the correctional system."
A Justice Department spokesman did not respond to a request for an explanation of Sessions' memo.
A history of zero-tolerance
Immigration advocates say they fear a return, and then some, to record levels of criminal prosecutions under the Obama administration for illegal entry and illegal re-entry, which now outnumber all other federal prosecutions.
That could include an expansion of "zero-tolerance" programs that saw prison as a way to deter illegal border crossings from those trying again after they were released and deported. One such program was called Operation Streamline, which rounded up migrants and brought them before judges in groups for guilty pleas.
But immigrant advocates say the threat of prison was not enough to stop migrants seeking to rejoin their families in the United States.
"What they've told us is, 'My entire life is here, so no matter what happens, I'm going back,'" said Carl Takei, senior staff attorney for the American Civil Liberties Union's National Prison Project.
The initiative, which began in 2005, has waned in recent years.
Sessions says that was a mistake.
At his Senate confirmation hearing in January, Sessions said he hoped to restore Operation Streamline, saying it has "great positive potential to improve legality at the border."
The result could be more immigrants getting sentenced to federal prison, straining a system that, despite having reduced the inmate population for three consecutive years, remains over capacity.
That could prompt a renewed reliance by the Bureau of Prisons on private facilities.
Sessions' statements, including the Feb. 21 memo, "imply that more beds will be needed in the future, and that's very concerning," said Edward Chung, vice president for criminal justice reform at the Center for American Progress, a liberal think tank.
But the interplay of Justice Department policies, prison logistics and inmate assignments are complicated, so it is difficult to predict how any single administration action will impact the population of private prisons.
Randy Capps, director of research for U.S. programs at the Migration Policy Institute, cautioned against assuming that more immigrants will pour into private prisons. While border agents could refer more immigrants for criminal prosecution, he said, it will be up to prosecutors and judges in those areas to move cases through the courts. That depends on having enough courtroom and detention space to accommodate them.
"One of the appeals of the private prison system is it's less expensive and is able to expand more quickly," Capps said. "But how and when and whether this translates into more people in prison, I don't know."
Private prisons' rise
The 20,625 so-called "criminal aliens" housed in Bureau of Prisons-contracted private prisons are low-security inmates, largely serving time for non-violent offenses: immigration crimes, drug crimes, fraud, burglary. A small percentage were convicted of weapons offenses, sex offenses or homicides, according to the bureau. They are predominantly Mexican.
Another 20,000 or so incarcerated people who are not citizens are doing time in traditional Bureau of Prison institutions, for similar or more serious offenses, the statistics show.
The companies that stand to benefit from the crackdowns are the same that run dozens of immigration detention centers across the country under contract with DHS ─ prison-like facilities that hold people who've been designated for removal from the United States.
Two that are publicly traded, The GEO Group and CoreCivic, formerly known as Corrections Corporation of America, have seen their stock climb since Trump's election, including a bump after Sessions' Feb. 21 memo. A third, smaller, private provider is Management and Training Corporation. Together, their contracts with the Bureau of Prisons totaled $639 million in fiscal year 2014.