Over the past few weeks, U.S. Immigration and Customs Enforcement posted notices on a federal contracting website seeking privately run jails to house some 4,000 immigration detainees. It sought bids for facilities in Chicago, Detroit, St. Paul, Salt Lake City and southern Texas, according to USA TODAY.

ICE made no public announcement of these plans prior to posting the notices. In a statement, it said the requests are not final. However, it says the requests are in line with a White House appeal for $1.2 billion in 2018 to increase capacity for detention to 48,000 people per day.

Currently, ICE has between 31,000 and 41,000 immigration detainees in custody every day. They are housed in local jails, private facilities and federal prisons.

"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 the head of Detention Watch Network.

According to USA TODAY, the Trump administration arrested 97,482 people it suspects of immigration violations between Jan. 22 and Sept. 9. That represents a 43 percent increase over the same time period in 2016.

During that same time period, ICE arrested 28,011 unauthorized migrants with no criminal record. That's a 179 percent increase over 2016 numbers. Previous administrations have typically prioritized immigrants for arrest only when they had committed serious crimes.

USA TODAY also cites Trump supporters as saying that these new detention facilities are necessary for handling an estimated 11 million unauthorized migrants in the U.S.

ICE may have chosen four cities because they are sanctuary cities

According to a spokesperson for the Center for Immigration Studies, which supports the Trump administration on immigration enforcement, there is a reason for choosing Chicago, Detroit, St. Paul and Salt Lake City. All four are sanctuary cities.

A sanctuary city (or state) is one that refuses to cooperate with some federal immigration requests. For example, a sanctuary city might not be willing to allow its police officers to enforce immigration law. The reason is that police need a trusting relationship with the immigrant community, which would be all but impossible if calling the police had immigration consequences.

"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," said the spokesperson for the Center for Immigration Studies.

The notices posted on the federal contracting website mark only the beginning of the contracting process. ICE will likely need substantial funding from Congress before any new facilities could be considered.