A group of Democratic senators led by Sen. Elizabeth Warren Elizabeth WarrenDimon: Wealth tax 'almost impossible to do' CNN's Don Lemon: 'Blow up the entire system' remark taken out of context Democrats shoot down talk of expanding Supreme Court MORE (D-Mass.) sent out letters Friday to three private immigration detention center contractors, demanding information on their allegedly poor conditions.

The Democrats wrote that it is “unclear” whether CoreCivic, The GEO Group and The Nakamoto Group are each “serving as a responsible steward of taxpayer dollars.”

CoreCivic and GEO operate detention facilities, while Nakamoto has a federal contract to inspect conditions in detention facilities.

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The senators pointed to a September report by the Department of Homeland Security (DHS) Office of Inspector General (OIG) that painted a stark picture of conditions in several ICE detention facilities.

Facilities were found to have old and moldy food, to lack adequate medical treatment and to present significant delays in getting detainees basic hygiene products, such as toilet paper.

In a letter to Jennifer Nakamoto, president of the Nakamoto Group, the senators outlined violations at Adelanto Detention Center, an Immigration and Customs Enforcement (ICE) facility in California operated by GEO and audited by Nakamoto. They said OIG inspectors found nooses made from bedsheets in 15 of 20 inspected cells.

“These reports on facility conditions are appalling and reveal serious problems at ICE — and with Nakamoto Group's inspections of those facilities,” they wrote.

The senators wrote that government auditors are nearly twice as likely to find violations as Nakamoto auditors, and that ICE employees told OIG that Nakamoto inspections are “very, very, very difficult to fail.”

The senators also focused on the treatment of immigration detainees, many of whom have not been charged with criminal behavior.

“The [DHS OIG] report also found that detainees were placed in solitary confinement or locked down in their cells without being told why, and for minor violations,” they wrote to GEO Group CEO George Zoley.

The letters come as Congress prepares for a lame-duck session where funding for DHS is expected to be a battle.

Warren was joined in the letters to the contractors by fellow Democratic Sens. Ron Wyden Ronald (Ron) Lee WydenDemocrats call for declassifying election threats after briefing by Trump officials Read Democrats' report countering Republicans' Biden investigation Top GOP senators say Hunter Biden's work 'cast a shadow' over Obama Ukraine policy MORE (Ore.), Kamala Harris Kamala HarrisHarris faces pivotal moment with Supreme Court battle Nearly 40 Democratic senators call for climate change questions in debates Joe Biden has long forgotten North Carolina: Today's visit is too late MORE (Calif.), Kirsten Gillibrand Kirsten GillibrandSunday shows preview: Justice Ginsburg dies, sparking partisan battle over vacancy before election Suburban moms are going to decide the 2020 election Jon Stewart urges Congress to help veterans exposed to burn pits MORE (N.Y.), Richard Blumenthal (Conn.), Cory Booker Cory Anthony BookerBipartisan praise pours in after Ginsburg's death DHS opens probe into allegations at Georgia ICE facility Democratic lawmakers call for an investigation into allegations of medical neglect at Georgia ICE facility MORE (N.J.), Ed Markey Edward (Ed) John MarkeyDemocrats shoot down talk of expanding Supreme Court Schumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Democrats see fundraising spike following Ginsburg death MORE (Mass.), Mazie Hirono Mazie Keiko HironoManchin defends Supreme Court candidate Barrett: 'It's awful to bring in religion' Democrats shoot down talk of expanding Supreme Court Democrats unveil plan declaring racism a public health issue MORE (Hawaii), Tom Udall Thomas (Tom) Stewart UdallLWCF modernization: Restoring the promise OVERNIGHT ENERGY: House Democrats tee up vote on climate-focused energy bill next week | EPA reappoints controversial leader to air quality advisory committee | Coronavirus creates delay in Pentagon research for alternative to 'forever chemicals' Senate Democrats demand White House fire controversial head of public lands agency MORE (N.M.), Jeff Merkley Jeffrey (Jeff) Alan MerkleyThe Hill's Morning Report - Sponsored by The Air Line Pilots Association - Trump, Biden renew push for Latino support Sunday shows - Trump team defends coronavirus response Oregon senator says Trump's blame on 'forest management' for wildfires is 'just a big and devastating lie' MORE (Ore.) and independent Sen. Bernie Sanders Bernie SandersOutrage erupts over Breonna Taylor grand jury ruling Dimon: Wealth tax 'almost impossible to do' Grand jury charges no officers in Breonna Taylor death MORE (Vt.).

They noted that CoreCivic and GEO made substantial political donations, including to President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE’s campaign and inaugural committee.

“Ultimately, CoreCivic's lobbying expenditures and donations have paid off,” wrote Warren to CoreCivic President and CEO Damon Hininger, using almost identical language in the letter to Zoley.

The senators asked CoreCivic and GEO to provide a list of facilities operated by the companies, inspection records, waivers granted by ICE on inmate safety regulations and any registered labor violations.

According to their letters, CoreCivic has received $217 million in federal contracts since 2017, while GEO has received $4 billion in contracts over the past decade, “including $560 million in the last two fiscal years.”

Nakamoto has received $55 million from ICE, and could potentially receive another $16 million, the Democrats' letters said.

Nakamoto was served with a laundry list of questions regarding its procedures, ICE supervision and regulations, inspection results to compare with OIG findings, government contracts, and an explanation of why the company's auditors “misrepresented detention conditions,” according to the OIG report.

Representatives for CoreCivic and GEO dismissed the allegations in the letters, particularly the suggestion that the companies' lobbying efforts and political donations are targeted at hardening the administration's immigration policies.

"Any impartial observer will see that this is more about politics than addressing the real and substantive challenges facing our immigration system," said Steve Owen, a spokesman for CoreCivic.

"CoreCivic did not make contributions to any presidential candidate or campaign during the last U.S. presidential election cycle. Our contribution to the inauguration events is consistent with our past practice of civic participation in and support for the inauguration process, including contributions to inauguration activities that took place under the previous administration," added Owen.

Pablo Paez, a spokesman for GEO, said the company "look[s] forward to addressing the many misconceptions and misrepresentations contained in [the letter]."

"While there has been a deliberate attempt to mischaracterize our role as a service provider to the federal government, our company has in fact never managed any facilities that house unaccompanied minors nor have we ever played a role in any policies related to the separation of families," added Paez.

Updated: 7:37 p.m.