Detained immigrants are given a unique identifying number called an Alien Registration Number ("A-number"), provided they do not already have one by virtue of their legal alien status, and are sent to a county, state, federal, or private prison, where they remain until deported.
There are four divisions within ICE which work to investigate illegal immigration, enforce immigration laws, and detain and deport offenders of these laws: Enforcement and Removal Operations (ERO), the Office of Investigations (OI), the Office of Intelligence (Intel), and the Office of International Affairs (OIA). ERO is the division that deals directly with the detention of immigrants. ERO, under ICE, operates eight detention centers, termed "Service Processing Centers," in Aguadilla, Puerto Rico; Batavia, New York; El Centro, California; El Paso, Texas; Florence, Arizona; Miami, Florida; Los Fresnos, Texas; and San Pedro, California. ICE also has contracts with seven private companies that run facilities in Aurora, Colorado; Houston, Texas; Laredo, Texas; Tacoma, Washington; Elizabeth, New Jersey; Queens, New York; and San Diego, California. Other facilities that house immigrant detainees include juvenile detention centers and shelters. However, the majority of immigrants are detained in state and local jails, which have contracts with ICE. At the end of the 2007 fiscal year the United States' immigration detention system comprised 961 sites.
In 2007, the program, Secure Communities, was created within ICE to identify criminal aliens, prioritize them based on the severity of the crimes that they committed, and transform the processes necessary to remove them by increasing efficiency. Secure Communities identifies undocumented immigrants with the use of modern technology, notably biometric identification techniques. These were first employed in Houston, Texas in 2008. As of April 2011, the Secure Communities' biometric sharing capability is being used in 1,210 of 3,181 jurisdictions (state, county, and local jails and prisons) in the U.S. Between 2008, when the program was started, through March 2011, 140,396 convicted criminal aliens have been booked into ICE custody resulting in 72,445 deportations. Under the Secure Communities program, the fingerprints of everyone arrested and booked are not only checked against FBI criminal history records, but they are also checked against DHS immigration records. If fingerprints match DHS records, ICE determines whether immigration-enforcement action is required, considering the immigration status of the alien, the severity of the crime, and the alien's criminal history. ICE then places a detainer on the individual, which is a request that the jail hold that person for up to 48 hours beyond their scheduled release, so that ICE can come to interview or to possibly take them into custody.

The Obama administration has been a big proponent of the pr…

Read more on Wikipedia

Immigration detention centers are managed by three agencies: Customs and Border Protection, Immigration and Customs Enforcement, and the Office of Refugee Resettlement.
Customs and Border Protection directly detains immigrants at the border and ports of entry. Facilities are designed for brief detention stays, after which migrants are usually turned over to Immigration and Customs Enforcement, or in the case of parent–child pairs and other children, to the Office of Refugee Resettlement. Under standard procedures, this detention should not exceed 72 hours, but in mid-2019, the average length of detention exceeded one week.

During the first half of 2019, the number of arriving immigrants on the U.S.–Mexico border increased greatly over prior years. The number of migrants held in CBP custody surged to 13,400 on March 27, 2019. In May and June 2019, between 14,000 and 18,000 people were held by CBP each night. On a press call, a Customs and Border Protection Commissioner McAleenan stated that, "A high number for us is 4,000. A crisis level is 6,000. 13,000 is unprecedented."

CBP detention facilities include the El Paso Del Norte Processing Center in El Paso, Texas, the "Ursula" Central Processing Center and Donna tent facility in McAllen, Texas, and the Border Patrol Station in Clint, Texas. During the 2019 surge in migrant arrivals, many CBP facilities held inmates in numbers far exceeding their planned capacities. Border Patrol agents converted numerous rooms and outdoor spaces into temporary holding facilities. The Department of Homeland Security's Office of Inspector General (OIG) reported that "the El Paso Del Norte Processing Center … does not have the capacity to hold the hundreds currently in custody safely, and has held the majority of its detainees longer than the 72 hours generally permitted." CBP facilities have been the sites of communicable infections including chicken pox, scabies, influenza, and lice.
Many of the detention centers housing immigrants are operated by private corporations who have contracts with ICE. The privatized model of detention, which is common within the United States' prison system, has raised several concerns. Without the government being directly involved, human rights abuses can go unmonitored and be difficult to uncover. The privatization model is based on profit maximization, meaning that more detainees result in more money for the private companies contracted to operate these facilities. While a cost-benefit argument is used to vouch for privatization, an attorney from the ACLU has stated, "It's much more expensive to detain people rather than supervise them to ensure that they appear for their removal proceedings and for deportation if necessary."

Several politicians have attempted to end or alter the terms of privatization. In 2015, Rep. Raul Grijalva introduced the Justice Is Not For Sale Act. Co-sponsored by thirty other Democrats, the bill aimed to effectively eliminate private detention centers and transfer their operation over to the federal government. Among some of the current complaints addressed in the bill were the cost of phone calls, which the bill required be capped, and to allow multiple telecommunication companies to provide phone service.

Rep. Adam Smith of Washington introduced a bill in 2015 entitled the Account…

Read more on Wikipedia

Several human rights organizations, including Amnesty International and the American Civil Liberties Union (ACLU), and a series of reports made by The New York Times have cited concerns with ICE's management of these detention centers. Reports refer to instances of human rights abuse and inadequate or unprofessional medical care in these detention facilities. Such reports have also publicized the death of several immigrants in detention and have accused ICE of covering up this information. ICE, in response, has released a list of 166 people who died under ICE detention between 2003–2016. ICE has publicly stated that the agency provides "state-of-the-art medical care" and "do[es] everything possible to maintain the best quality of life for the detainees in…custody." In May 2008 the Detainee Basic Medical Care Act of 2008 (H.R. 5950) was introduced to the United States Congress by Representative Zoe Lofgren (D-CA), though no further steps have been taken to enact the bill.

In addition to their condemnation of the conditions at immigration detention centers, various human rights groups and news sources have also criticized the high costs necessary to sustain ICE's detention infrastructure. ICE's annual budget is roughly 2.5 billion for its detention and deportation duties. President Obama's fiscal year 2017 budget request would reduce the bed quota to 30,913 detention beds: 29,953 adult beds at an average rate of $126.46 per day and 960 family beds at an average rate of $161.36 per day. Through fiscal year 2016, the bed quota remains at 34,000.

It has been reported that only a small percentage of the population of detained immigrants have committed crimes. However, of the 32,000 immigrants in ICE detention on January 25, 2009, 18,690 had no criminal convictions, including illegal entry.

Protests by detained immigrants have taken place at several facilities, including the Varick Federal Detention Facility in Manhattan. One hundred detainees at the Varick site, all of whom had no criminal charges, sent a petition in October 2008 to the New York City Bar Association detailing human rights abuses and substandard conditions and asking for legal help. On January 19, 2010, detainees at the same site initiated a hunger strike against the conditions of detention. The strike was broken up by a SWAT team that allegedly "used pepper spray and "beat up" some detainees, took many to segregation cells as punishment and transferred about 17 to immigration jails in other states." In response to the report, the New York City Bar Justice Center began the NYC Know Your Rights Project. This initiative combines the efforts of the City Bar Justice Center, the Legal Aid Society and the New York chapter of the American I…

Read more on Wikipedia