The US Department of Justice and embattled mayor Rahm Emanuel are under mounting pressure to investigate allegations of what one politician called “CIA or Gestapo tactics” at a secretive Chicago police facility exposed by the Guardian.

Politicians and civil-rights groups across the US expressed shock upon hearing descriptions of off-the-books interrogation at Homan Square, the Chicago warehouse that multiple lawyers and one shackled-up protester likened to a US counter-terrorist black site in a Guardian investigation published this week.

As three more people came forward detailing their stories of being “held hostage” and “strapped” inside Homan Square without access to an attorney or an official public record of their detention by Chicago police, officials and activists said the allegations merited further inquiry and risked aggravating wounds over community policing and race that have reached as high as the White House.

Caught in the swirl of questions around the complex – still active on Wednesday – was Emanuel, the former chief of staff to Barack Obama who is suddenly facing a mayoral runoff election after failing to win a majority in a contest that has seen debate over police tactics take a central role.

Emanuel’s office refused multiple requests for comment from the Guardian on Wednesday, referring a reporter to an unspecific denial from the Chicago police…