An investigation by Senate Democrats into the operation of ICE detention facilities by private prison companies just keeps turning up more questions.

That’s according to the latest letter from Massachusetts Sen. Elizabeth Warren and 10 other Democratic caucus members, including fellow 2020 presidential candidates Bernie Sanders of Vermont, Kirsten Gillibrand of New York, Kamala Harris of California and Cory Booker of New Jersey.

“Based on these concerns, it appears that there is no entity in charge of and taking responsibility for the conditions at ICE detention facilities,” the senators wrote.

“The response to our investigation was distressing, revealing a failure at multiple levels. The two private contractors we asked for information refused to provide it, indicating that doing so was the responsibility of ICE,” they said. “And the Nakamoto Group responded to [the Homeland Security Department’s Office of Inspector General’s] identification of weaknesses in its inspection methodologies with information that OIG determined to be false and misleading.”

The latest letter, shared first with CQ Roll Call, is the next step in an investigation started in November after reports of makeshift nooses in detention cells in privately operated facilities.

Warren led a group of 11 Senate Democrats in sending a letter dated Monday to Immigration and Customs Enforcement acting Director Matthew T. Albence asking the agency to follow up on exchanges of letters between the senators and the Homeland Security inspector general, prison contractors CoreCivic and GEO Group, and Nakamoto, an ICE contractor that conducts inspections.

Jenni Nakamoto, the president of the Nakamoto Group, wrote in a December reply to the Senate’s initial queries that her company completed inspections under terms outlined by Immigration and Customs Enforcement.

“If another entity observed an issue while they were at the facility, that was not ongoing during our inspection, we of course cannot be accountable for that,” Nakamoto wrote. 

John V. Kelly, the acting DHS inspector general, alleged in an April 4 letter directed to Warren that the contractor’s response contained “errors and misrepresentations” about the IG’s findings.

“During the course of our inspection, we observed Nakamoto’s practices conducting inspections at two facilities, thoroughly documented every observation, and verified observations against the checklist records and reports Nakamoto completes. We concluded that because the inspection scope — determined by ICE — is too broad, the Nakamoto inspections are not thorough,” Kelly wrote in the IG response letter.

The senators were both concerned about the scope of the Nakamoto Group inspections and the initial response letters from the two facility operators.

“CoreCivic and GEO Group failed to address our serious and substantive concerns and those identified by OIG,” the senators wrote. “We are therefore asking that you provide us with information about each immigration detention facility operated by CoreCivic and GEO Group, their inspection records and results of audits, information about deficiencies cited in the reviews, as well as evidence that those deficiencies have been or are being addressed.”

Among the questions that the senators are now asking for responses from Albence are what ICE has already done to respond to concerns raised by the inspector general about the Nakamoto-led inspections, which have been described as “useless” because in part they are not random, surprise inspections.

The senators also want details of all waivers that ICE has granted to the CoreCivic and GEO Group facilities for potential violations of detention condition standards .

“For each waiver, please provide all correspondence with the contractor, including the initial waiver request, the ICE decision on the waiver, and any additional communications related to the waiver request,” the senators wrote. “Please provide an overview of the process by which ICE reviews and grants waivers from auditors for cited deficiencies.“