A stinging report released on Thursday called on the Obama administration to reverse course and stop detentions of women and children who entered the United States illegally but might qualify for asylum.
The U.S. Commission on Civil Rights said it found evidence that the federal government "was interfering with the constitutional rights afforded to detained immigrants," including their access to legal representation.
The commission, created in 1957, describes itself as an independent, bipartisan federal agency that helps develop civil rights policy and enhance enforcement of civil rights laws.
A year ago, President Barack Obama responded to a "humanitarian crisis" unfolding on the U.S. southwestern border with Mexico, as tens of thousands of children - some traveling with parents and others alone - arrived from El Salvador, Guatemala and Honduras.
Among steps he took were a rapid expansion of detention facilities for migrant women and children. It marked a departure from previous practices of largely tracking the immigrants with electronic ankle bracelets and telephone check-ins, which immigration rights groups argued were effective and far less costly.
The arrivals of undocumented children, many claiming they were fleeing drug-related violence, have slowed compared with last year. But a steady stream remains, placing huge demands on Department of Homeland Security border enforcement and immigration courts.
Some Republican presidential candidates have called for erecting a border wall and deporting all 12 million undocumented residents, including children, now thought to be in the United States.
According to the lengthy report from the U.S. Commission on Civil Rights, some of the detention facilities "have failed to comply with DHS standards for medical care, including ignoring serious medical conditions."
The commission said it had received reports that children were abused while in custody of federal agents and that families suffered poor conditions while held at Customs and Border Protection stations.
The Department of Homeland Security is reviewing the report, spokeswoman Marsha Catron said.
"Recognizing the sensitive and unique nature of detaining families, we have made significant changes to the family residential facilities," Catron added.
"Of note, we are transitioning these facilities into short-term processing centers where individuals who claim fear of return to their countries can be interviewed for asylum and other humanitarian protections."
From Oct. 1, 2014, to July 31, 2015, 30,862 unaccompanied children and 29,407 families were apprehended at the southwestern border. Those numbers were down by 51 percent and 53 percent, respectively, from the same period the previous year, government statistics showed.
(Reporting by Richard Cowan; Editing by Lisa Shumaker and Leslie Adler)