Donald Trump’s plan for the mass deportation of undocumented people and a related expansion of US detention facilities is getting an unexpected head start from Joe Biden – and private prison companies are already cashing in.

The Biden administration has, for the past year, been in the process of extending contracts for private sector immigration jails across the US and exploring options for expanding detention capacity, a Guardian investigation has found.

This is despite both the US president’s previous statements opposing such private jails and the fact that many facilities have a reputation for inhumane conditions.

For this story, the Guardian reviewed hundreds of pages of contracting documents, government statements, inspection reports and private prison contractor calls with investors.

The investigation reveals that during this final year of Biden’s presidency, the Immigration and Customs Enforcement (Ice) federal agency has been working on extending at least 14 contracts with private prison companies to run a number of immigration detention centers across the country, against significant opposition from Democrats and advocates.

All those facilities have fallen under intense scrutiny by, variously, federal government watchdogs, the US Senate and advocates, examining allegations of troubling and dangerous detention conditions. But the White House has resisted protests from congressional Democrats and campaign groups about the scale and standard of detention.

“The Biden administration has increased the number of people in detention, doubled down on anti-immigrant, Trump-era deterrence policies, and expanded detention capacity in direct conflict with its own promises to end for-profit incarceration and bring fairness to the immigration system,” said Silky Shah, executive director of Detention Watch Network, a national advocacy working with detained immigrants.

Instead of reassuring and pre-emptively protecting many terrified families and workers among the 11 million undocumented people in the US, the Biden administration has been working on contracts for more detention beds. These plans may not all get finalized, but private prison companies are enthusiastic about expansion opportunities now, as well as next year.

“Let us be clear: Trump’s return to the White House, and his unprecedented mass deportation and immigration detention agenda will be catastrophic for all Americans,” Shah said. Families face being split up and communities turned upside down, she said. That’s apart from huge cost and economic production implications.

“Biden now has a last chance to take decisive action to prevent catastrophe for millions of people and avoid handing the keys to an expanded and inhumane detention and deportation system to the next administration,” she added. There are no signs of such action so far in Biden’s waning weeks.

Private prison contracts are negotiated by the number of “beds” in a facility. Documents reviewed by the Guardian show that Ice is in talks to add bed space in many locations, including in New Jersey, the midwest and several west coast cities that would add capacity to detain a total of at least 4,850 more people, amid plans to create new facilities or expand existing ones. That’s in addition to the almost 39,000 people currently held in federal immigration custody.

Biden’s push is already “the greatest level of procurement activity we’ve seen with Ice in over a decade, demonstrating the continued need for additional detention solutions in various locations throughout the US”, Damon Hininger, the president and CEO of private contractor CoreCivic, said during a post-election call with investors.

CoreCivic and GEO Group, the two biggest private prison contractors, reckon that the incoming Trump administration will be a further boon for their businesses and they are ready to assist the Republican president-elect’s mission to deport millions. People will likely be detained for a period between being arrested and expelled, as legal and logistical processes play out.

“This is, to us, an unprecedented opportunity to assist the federal government and the incoming Trump administration toward achieving a much more aggressive immigration policy with regard to interior enforcement and border enforcement, and the removal of criminal aliens predominantly from this country,” said Brian Evans, the CEO of GEO Group, during a call with investors after Trump’s victory.

The White House and Ice did not respond to requests for comment before publication.

Private prison contractors GEO Group, LaSalle Corrections, Akima and Abyon LLC did not respond to requests for comment before publication. CoreCivic sent a statement.

“CoreCivic is committed to providing safe, humane and appropriate care for the people in our facilities,” a CoreCivic spokesperson said. “Our facilities adhere to all Ice standards and are monitored by Ice officials every day. Our facilities are also audited regularly and without notice several times a year, and they are routinely visited by elected officials, attorneys, families and volunteers.”

In mid-November, nearly 200 organizations wrote a letter to Biden and to the homeland security secretary, Alejandro Mayorkas, urging them to close detention centers and halt expansion efforts before Trump takes office.

“Facilities across the system have documented instances of physical abuse, inadequate food and water, negligent or abusive medical care, unsanitary conditions, failed plumbing and more. Avoidable deaths in Ice detention centers are at record numbers,” the letter said.

During his 2020 presidential campaign, Biden promised a significant shift from Trump’s first term hardline immigration policies, including promising an end to privately-run detention centers.

“No business should profit from the suffering of desperate people fleeing violence,” Biden had said. But four years on, the administration continues the rightward turn away from its 2020 promises and some early actions. Especially in the run-up to the 2024 elections, with immigration a central issue and a crackdown on asylum, Ice leaned in to expanding immigration detention.

The 110 Ice facilities throughout the country typically detain undocumented people going through deportation proceedings as well as recently-arrived asylum seekers fighting their case to stay in the US but whom the government has barred from release while awaiting court dates. Although immigration detention is a civil not criminal system, conditions inside Ice detention centers are often indistinguishable from jails and prisons. Some are, indeed, former jails and prisons run by for-profit companies on government contracts.

Texas has the most Ice detainees, followed by Louisiana, then California.

Contract documents show that the Biden administration aims to add detention beds in Phoenix, Arizona, El Paso, Texas, Seattle and San Francisco, as well as other locations, and that Ice is now reviewing companies’ bids.

“It is deeply concerning that Ice would expand private detention facilities now – just months before the inauguration of a president-elect who has promised to terrorize immigrant communities by detaining and deporting millions of people,” Senator Cory Booker said in a statement to the Guardian.

He added: “I will continue to fight to ensure that detained immigrants are treated with dignity and that we value the wellbeing of immigrant communities in New Jersey above the share price of for-profit detention corporations.”

Earlier this month Trump confirmed he would use the military to assist in mass deportations and adviser Stephen Miller said military funds would be used to create new “vast holding facilities” to process migrants for deportation.

Meanwhile, Biden has not backed away from his tougher stance.

“It’s really just a step backwards,” said Esmeralda Santos, a lead organizer with the Inland Coalition for Immigrant Justice, a group helping migrants in detention, adding: “We’re seeing Biden replicate some of the policies Trump was interested in and it’s really discouraging to see that. I thought we could have more faith in Biden. He let us down, honestly.”

In September, a federal watchdog report on 17 unannounced inspections at Ice detention facilities, spread across the country, found many horrible conditions.