WASHINGTON (AP) — A “near-total secrecy” surrounding deportation flights and the use of full-body restraints onboard is raising “serious human rights concerns,” a group of 11 Democratic U.S. senators wrote in a letter Thursday to top immigration officials.

U.S. Sen. Chris Van Hollen of Maryland called upon U.S. Immigration and Customs Enforcement to provide a full accounting of its air operations and to stop using the black and yellow restraints known as the WRAP until the agency explains its policies for the device and resolves other questions about its use on immigration detainees.

“I think it’s very problematic,” Van Hollen told The Associated Press. “They want to keep the public in the dark.”

The senators’ letter cites an AP investigation this month that revealed several examples of ICE using the device on people — sometimes for hours — on deportation flights dating to 2020. Van Hollen was joined by U.S. Sens. Elizabeth Warren of Massachusetts, Cory Booker of New Jersey, Alex Padilla of California, Tammy Duckworth of Illinois, and six others.

The WRAP is the subject of several federal lawsuits likening incorrect usage of the device to punishment and even torture. Advocates have expressed concern that ICE is not tracking the WRAP’s use as required by federal law when officers use force, making it difficult to discern exactly how many people are being subjected to the restraints.

“When an organization like DHS doesn’t want transparency, it’s because they don’t want people to know what they’re doing,” Van Hollen said, referring to the U.S. Department of Homeland Security, ICE’s parent agency.

U.S. Rep. Delia Ramirez, D-Ill., is also working on a bill to rein in the agency’s use of the WRAP. “ICE’s use of full-body restraints to immobilize detained individuals raises serious concerns about the safety, dignity, and human rights of those under their jurisdiction,” Ramirez said.

DHS has not answered detailed questions from the AP about the use of the WRAP. DHS spokesperson Tricia McLaughlin previously stated that ICE’s practices “align with those followed by other relevant authorities and is fully in line with established legal standards.”

The AP found that since 2015, DHS has paid the manufacturer of the WRAP, Safe Restraints Inc., $268,523, with two Trump administrations accounting for around 91% of that spending. ICE has not provided records documenting the usage of the WRAP despite repeated requests, raising further questions concerning the agency's accountability and oversight.

Concerns about the brutal tactics employed by ICE continue to mount, leading to calls for greater oversight and reform to protect the rights of detainees.