At least 3,500 Americans have been detained inside a Chicago police warehouse described by some of its arrestees as a secretive interrogation facility, newly uncovered records reveal.

Of the thousands held in the facility known as Homan Square over a decade, 82% were black. Only three received documented visits from an attorney, according to a cache of documents obtained when the Guardian sued the police.

Despite repeated denials from the Chicago police department that the warehouse is a secretive, off-the-books anomaly, the Homan Square files begin to show how the city's most vulnerable people get lost in its criminal justice system.

The Chicago police department has maintained – even as the Guardian reported stories of people being shackled and held for hours or even days, all without legal access – that the warehouse is not a secret facility so much as an undercover police base operating in plain sight. "There are always records of anyone who is arrested by CPD, and this is no different at Homan Square," the police asserted in a March statement.