Amid the Trump administration's efforts to arrest people living in the country illegally, the Department of Homeland Security is looking at locations for five new detention centers around the country that could hold thousands of detainees.

According to a request for information posted to a federal contracting website on Oct. 12, Immigrations and Customs Enforcement is considering sites near four cities where it has offices: Chicago, Detroit, Salt Lake City and St. Paul, Minn. Last month, the agency put out a similar request to identify a possible detention site in South Texas.

In a statement to NPR, ICE said the postings are part of its regular "market research" and that it was soliciting feedback on possible vendors, locations and facilities. The agency said it had no restriction on who could respond to its request, "private company or otherwise."

ICE reports the average daily population in its detention facilities was a little more than 38,000 for the 2017 fiscal year. The president's 2018 budget plan requests an increase of $1.2 billion in funding for detention beds, to support an average population of over 48,000 adults.

The ACLU announced Thursday that it was joining with legal services and immigrants' rights organizations in asking ICE not to open the new detention facilities.

"ICE's intention to expand detention in areas surrounding four of the nation's largest cities is deeply disturbing," said Lorella Praeli, director of immigration policy and campaigns at the ACLU. "This move represents further action by the Trump administration to target long term residents, including Dreamers, asylum seekers fleeing persecution in their home countries, and other immigrant communities. The ICE detention system is already notorious for inhumane and abusive conditions as well as lack of transparency and accountability."

Arrests by ICE have spiked since the beginning of the Trump presidency. ICE reported that it arrested 41,000 people in just the first 100 days after the president signed his executive order on border security and immigration enforcement.

But despite a 43 percent increase in immigration arrests since Trump's inauguration, ICE actually deported fewer people in the 2017 fiscal year than in the previous year. According to The Washington Post, that's because the group of people easiest to deport, those caught sneaking over the border illegally, dropped dramatically after Trump took office.

The ICE website lists dozens of detention facilities around the country, but many of them are county jails. In contrast, ICE's posting for a South Texas site seeks a facility large enough for 1,000 adult detainees. ICE is interested in facilities where it could detain 400 to 1,200 people in Chicago, and 200 to 600 detainees in each of Detroit, St. Paul and Salt Lake City.

As NPR's John Burnett has reported, the Department of Homeland Security relies on privately owned facilities to house more than 70 percent of its detainees. The Justice Department announced last August that it wanted to phase out private prisons, but because ICE is part of DHS, that rule does not apply to immigrant detention centers.

The U.S. has an immigration court backlog of more than 600,000 cases — or more than 2,000 pending cases for every immigration judge. That means those who are arrested for being in the country illegally could sit in ICE's detention centers for years before they are deported.