The Justice Department's plan to phase out its use of private prisons — the result of declining inmate populations and concerns about safety and security — ended this week without ever really taking effect.

The reason is a new administration that has called for a crackdown on what it sees as a rise in crime.

That crackdown could lead to more arrests, which in turn could result in more people in prison. Which, presumably, is why Attorney General Jeff Sessions rescinded a six-month-old Obama administration directive that sought to curtail the government's use of private prisons.

Attorney General Jeff Sessions holds a meeting with the heads of federal law enforcement components at the Department of Justice in Washington on Feb. 9, 2017. Susan Walsh / AP

Sessions said in a Feb. 21 memo that the Obama move had "impaired" the U.S. Bureau of Prison's "ability to meet the future needs of the federal correctional system."

Those "needs" are not yet clear. Asked for an explanation, a Justice Department spokesman said only that Sessions' move "returns discretion to the professionals at BOP who are in the best position to evaluate their needs."

The memo itself suggests that the attorney general is concerned about running out of available space in the 122 prisons the BOP runs itself — although current trends run opposite.

The number of people in federal prison has been declining since 2013, and the 189,078 currently there is the lowest in a decade. That includes 21,366 people in private prisons, just 12 percent of the total population.

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Compared to the more than 2 million people estimated to be behind bars across the American justice system, the private prisons under BOP contract play a bit role in a system that contributes to one of the world's highest incarceration rates.

So, why does Session's memo matter?

Here are some additional points to put the issue in context:

The relationship goes back decades

The Bureau of Prisons first turned to privately operated prisons in 1997 to help alleviate overcrowding. The partnership grew steadily, focusing primarily on low-security inmates designated as "criminal aliens" — non-citizens who've committed a crime — with less than eight years left on their sentences. In fiscal year 2014, the contracts were worth about $639 million, up from $562 million in fiscal year 2011.

Private prisons mostly benefit two publicly traded companies

Those companies are CoreCivic (which until recently was known as Corrections Corporation of America), and The Geo Group. The two companies, both generous contributors to President Trump, run the majority of the BOP's contracted prisons, as well as dozens of other facilities used to detain immigrants who are in the country illegally.

Related: Corrections Corp. and GEO Group Prison Stocks Surge On Trump Win

The companies' fortunes climb and fall with federal criminal justice policies. Their stocks plunged after Obama's deputy attorney general issued the phase-out memo on Aug. 18, and shot up after Donald Trump, who'd called for more private prisons, won election on Nov. 8. Since then, companies' share prices have been steadily rising — and they enjoyed a bump from Sessions' memo.

The federal government has predicted that private prison populations will drop

Even before the August Obama administration memo, the Justice Department saw a weakening reliance on private prisons. The memo's author, then-Deputy Attorney General Sally Yates, noted that the BOP was already cutting contracts with for-profit companies, so much so that it planned to stop sending inmates to three private prisons, shrinking the private-prison population to less than 14,200 by May 1 — less than half of what it was in 2013.

There are a lot of documented problems with private prisons housing federal inmates

Yates' memo followed a report by the Justice Department's Office of the Inspector General that said privately run prisons had more safety and security incidents — including more assaults by inmates on each other and by inmates on prison staff — than those operated by the Bureau of Prisons. The companies, GEO and CoreCivic, responded in letters that the discrepancies were attributable to the fact that they primarily housed a wide array of foreign nationals, including gang members.

Officers stand on the roof of and outside the gates at the Adams County Correctional Center in Natchez, Miss., during an inmate disturbance at the prison, May 20, 2012. Lauren Wood / AP

In December, just before the Obama administration left power, the inspector general released another report, which focused on CoreCivic's prison in Natchez, Mississippi, where a May 2012 prison riot, triggered by complaints of poor conditions, ended with a correctional officer's death. The December report found the prison "was plagued by the same significant deficiencies." CoreCivic responded that it had corrected many of its problems, and took issue with some of the methods used by government auditors.

Investigative journalists have uncovered other alleged abuses. A series in the Nation documented dozens of deaths that the magazine attributed to substandard care. And a reporter for Mother Jones went undercover as an officer at a private prison in Louisiana, vividly detailing many of the problems documented by the government audits.