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As the use of solitary confinement in immigration detention centers appears to increase under the Biden administration, some Democratic lawmakers are demanding an end to the practice — or at least the creation of rules that would limit it — accusing the federal government in a letter of being "in clear violation of international norms."

U.S. Immigration and Customs Enforcement "has isolated individuals for months and even years, used solitary as punishment for minor infractions, and placed in solitary vulnerable individuals, including those with mental health conditions," Massachusetts sens. Ed Markey and Elizabeth Warren and Illinois Sen. Dick Durbin wrote in a letter TKday to Homeland Security Secretary Alejandro Mayorkas and ICE acting Deputy Director Patrick Lechleitner.

"Existing ICE guidelines have clearly failed to protect the rights of detained noncitizens," the senators added.

ICE statistics show the agency detains more than 38,000 people each day, an increase of about 15,000 since President Joe Biden took office in 2021.

The senators cited a study released last month that found 1,106 uses of "segregation" — informally known as solitary confinement — in the third quarter of 2023, up 61% from a year prior. Researchers at Harvard University and the nonprofit group Physicians for Human Rights based the analysis on ICE's own data, and also established the agency placed people in solitary confinement over 14,000 times in the past five years with an average duration of 27 days — "well exceeding the 15-day threshold that United Nations human rights experts have found constitutes torture."

The senators said they are especially concerned about detainees in the most vulnerable populations, including LGBTQ people and those with mental health and chronic medical conditions.

Medical and mental health remain the largest reasons why immigration detainees are placed in segregation, according to ICE data, followed by disciplinary and protective custody issues.

"ICE should discontinue the use of solitary confinement as a form of discipline and establish meaningful alternatives to solitary confinement that would phase out its use," the Democratic lawmakers wrote. "At a minimum, ICE must issue binding rules limiting its use of solitary confinement and follow them."

Senators asked Mayorkas and Lechleitner for anwsers to several questions, including what steps have been taken to limit solitary confinement, what is the breakdown of time spent in it and by vulnerable population, and what is the agency doing to respond to recommendations from government accountability and oversight offices to ensure clear and consistent policies for segregation.

DHS and ICE did not immediately respond to a request for comment about the senators' letter.

In 2013, ICE issued a directive that the use of segregated housing must be carefully weighed.

"Placement in segregation should occur only when necessary and in compliance with applicable detention standards," the agency said. "In particular, placement in administrative segregation due to a special vulnerability should be used only as a last resort and when no other viable housing options exist."

The agency also agreed to protect transgender people in a 2015 memorandum.

Immigration advocacy groups remain dismayed over the use of solitary confinement and say the issue is taking greater importance ahead of an upcoming election that is on course to see a rematch between Biden and former President Donald Trump.

In 2019, an NBC News investigation in parternship with the International Consortium of Investigative Journalists and other news organizations found a widespread use of solitary confinement for immigrant detainees in ICE custody under both the Obama and Trump administrations.

Concerns continue in detention centers under the Biden administration, with detainees in Louisiana last year telling NBC News about the threat and use of solitary confinement as punishment. The president has been criticized during his tenure about the increased use of so-called restrictive housing in federal prisons as well, despite a campaign pledge to end solitary confinement except for "very limited" reasons. Bills were introduced last year in the U.S. Senate and House by Democrats to largely ban the practice on federal inmates and detainees.

Jesse Franzblau, a senior policy analyst at the National Immigrant Justice Center, which offers legal representation for detainees, said he believes what has been happening in federal facilities must change because it is "arbitrary."

"There's already weak standards in terms of human rights and basic humanity, and there's not a lot of oversight and accountabilty from staff just throwing people into solitary and them not having the opportunity to get out," Franzblau said. "This is how it plays out on the ground when members of Congress say the government is violating its own policies."