A dozen senators on Friday urged the US Department of Homeland Security to reform its system of placing detained immigrants in solitary confinement, referring to the practice as a “clear violation of international norms” in a heated letter.

The senators wrote that in recent years, solitary confinement has been on the rise in Immigration and Customs Enforcement (ICE) detention centers, saying the practice had increased by 61% between 2022 and 2023, and that between 2018 and 2023, ICE officials ordered some 14,000 periods in solitary.

The practice entails leaving detainees in cells without human contact for upwards of 22 hours a day, the senators wrote, often for minor infractions such as the use of profanity, and in some cases, as a means of responding to mental health needs.

Rule 43 of the United Nations Standard Minimum Rules for the Treatment of Prisoners prohibits “prolonged solitary confinement” — described as solitary confinement for periods exceeding 15 days — under all circumstances.

ICE is tasked with enforcing immigration laws within US borders. Its broad mandate extends to the identification, arrest, and removal of immigrants who are present in the country illegally or have violated immigration laws, as well as combating human trafficking and smuggling, and preventing terrorism through immigration pathways. ICE’s operations have been a subject of significant debate and controversy, including for the detention and deportation of children. A recent report by Physicians for Human Rights criticized ICE for its use of solitary confinement, which it describes as causing a broad range of mental health impacts, from PTSD to heightened suicide risk.