The Trump administration is planning an increase in federal immigration jails across the country for the thousands of additional undocumented immigrants its agents are arresting.

In recent weeks, the Immigration and Customs Enforcement (ICE) agency has put out requests to identify privately-run jail sites in Chicago, Detroit, St. Paul, Salt Lake City and southern Texas, according to notices published on a federal contracting website. It did not publicly announce its plans to house 4,000 more detainees at the facilities.

The detention expansion would represent the latest step in President Trump's efforts to crack down on illegal immigration. From Jan. 22 through Sept. 9, the agency arrested 97,482 people suspected of being in the country illegally, a 43% increase over the same time period in 2016 under President Barack Obama, according to the latest ICE figures.

During the same span, ICE arrested 28,011 undocumented immigrants without a criminal record, a 179% increase from the same period in 2016, when the Obama administration mainly went after those who committed serious offenses.

Currently, ICE houses anywhere between 31,000 and 41,000 detainees each day in federal prisons, privately-operated facilities and local jails.

In a statement, ICE said the requests for new jails are far from finalized. But the agency said they are in line with the White House's request for an additional $1.2 billion in the 2018 budget to increase detention capacity to 48,000 detainees a day.

"The administration is doing everything it can on all fronts to detain and deport as many people as possible, and to criminalize as many people as possible," said Silky Shah, executive director of Detention Watch Network, which advocates against incarceration for immigration violators. It is "a very nativist and xenophobic position."

Trump supporters say the new jails are necessary to tackle an estimated 11 million undocumented immigrants.

Jessica Vaughan, director of policy studies at the Center for Immigration Studies, which backs Trump's immigration enforcement, noted that four of the cities identified for new jails — Chicago, Detroit, St. Paul and Salt Lake City — are all "sanctuary cities."

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That term refers to more than 300 local governments that limit their cooperation with federal immigration agents. The Trump administration has tried to withhold federal funding from those cities, which have fought back in court.

One of the core disputes is that some cities refuse to detain undocumented immigrants in their local jails for federal immigration agents.

"ICE cannot rely on local law enforcement agencies to cooperate with them in holding deportable criminal aliens, so they have to acquire their own space that they control," Vaughan said. "This is very encouraging."

ICE still has a long way to go before it can open any facilities. The notices invite private companies to provide information on possible locations, and whether it would be necessary to build new facilities or renovate existing ones.

The notices, known as Requests for Information, represent the start of the federal contracting process, and do not yet ask for cost estimates. But a jail contract announced in April provides some context.

The federal government announced that it had awarded a $457 million contract to GEO Group, one of the leading private prison contractors in the country, to build and operate 1,000-bed immigration detention center outside of Houston. Carl Takei, a staff attorney with the ACLU's National Prison Project, said that kind of price tag means that ICE will most likely need new funding from Congress to build any more prisons.

"To me, this is a signal that ICE wants to be ready, pen in hand, to sign new detention contracts as soon as Congress appropriates more money for detention," Takei said.

The push to expand detention space in the Midwest shows how the administration is not solely focused on undocumented immigrants crossing the southwest border with Mexico.

Last month, ICE conducted a nationwide sweep dubbed "Operation Safe City" that resulted in more than 450 arrests of people living in Baltimore, Chicago, Denver, New York, Philadelphia, Seattle, Washington, D.C. and Massachusetts.

"The Obama administration focused heavily on apprehending people on the border, but the Trump administration is targeting people in U.S. communities very far from the border," Takei said. "And because they are targeting cities far from the border, they are looking for detention space in areas where historically they haven’t had as much detention space."

The most likely candidates to land any new contracts are two private prison groups that have dominated the market and donated heavily to Trump.

Florida-based GEO Group donated at least $475,000 to Trump's inauguration festivities and a Super PAC that supported Trump's presidential campaign. Tennessee-based CoreCivic, the other major private prison contractor in the U.S., gave $250,000 to support Trump’s inauguration.

The prospect of new prisons has driven up the value of both companies since Trump's election, partly because 65% of detainees held by the Department of Homeland Security are in privately run facilities. Since Election Day, GEO Group's stock price has increased 63% and CoreCivic's has risen 81%.

Neither company responded immediately to requests for comment.