An exterior view of the Chicago detention facility known as Homan Square. Screengrab/Philipp Batta for The Guardian Chicago's police department is detaining US citizens for days on end in a secret compound where suspects have no contact with the outside world, the Guardian reports today.

Lawyers have compared the off-the-books interrogation warehouse in Chicago's Homan Square neighborhood to the CIA's so-called black sites offshore that are used to interrogate terrorists.

Police at the site in Chicago reportedly carry heavy military gear, and huge armored tanks are parked outside.

"There are usually questions about whether these arrests are justifiable or constitutional," Anthony Hill, a criminal defense attorney, told Business Insider. "Suspected criminals are just picked up and thrown into the back of unmarked cars by police officers wielding assault rifles and wearing bulletproof vests. Describing the process as highly militarized would be fair."

"It's a black hole," Hill added.

Suspects brought in for questioning don't get read their Miranda rights, according to the Guardian, which also reported that they don't get access to lawyers either. People are reportedly shackled for hours on end at the facility before being taken to a police station, booked, and formally charged.

Hill noted that when arrests are questionable, police will often take suspects to Homan Square instead of to a police station to avoid having a record of an arrest ever being made.

What actually goes on inside the facility — referred to by lawyers and civil rights activists simply as "Homan Square" — is largely a mystery to those on the outside. However, the Guardian reports that it's clear the facility is used as a place to interrogate individuals out of view of law enforcement's regular chain of command.

Lawyers who can't find their clients at police stations often try to find them at the warehouse.

"This place is sort of an open secret among attorneys that regularly make police station visits," Chicago lawyer Julia Bartmes told the Guardian. "If you can't find a client in the system, odds are they're there."

Lawyers who show up at the warehouse to speak to their clients, however, are frequently turned away by guards who tell them that they are jeopardizing an undercover police operation, or blatantly lie and tell lawyers that their clients are not there. Hill claims to be one of the few lawyers who has actually been inside the facility, and this is only because he snuck in, taking advantage of the few precious moments when security guards were away from their post. "The interrogators seemed shocked — and not happy — that I had managed to make it inside," Hill said.

Clients who want to call their lawyers are denied that option, too, even though police directives stipulate that detainees be allowed to make phone calls and receive visits from their attorneys. "As far as people going off the radar and being denied access to lawyers," Hill said, "it sounds like it's still a major problem."

Brian Jacob Church, who was arrested and held at Homan Square for inciting 'mob violence' during a protest, was later cleared of all charges. Screengrab/Philipp Batta for The Guardian Brian Jacob Church — who was arrested at a protest in 2012 and detained at Homan Square for 17 hours in handcuffs and ankle restraints — wasn't sure if he would ever be released. "Essentially, I wasn't allowed to make any contact with anybody," Church told the Guardian.

Part of the reason why there hasn't been more scrutiny, Hill noted, is because former detainees are not speaking out about their experiences for fear of retribution.

The whole thing is eerily reminiscent of the post-9/11 military operations that continue to take place at US-run detention centers around the world, such as Guantánamo Bay in Cuba and, previously, Abu Ghraib in Iraq. Unlike at Guantánamo and Abu Ghraib, however, not all detainees at Homan Square are suspected terrorists.

Many of them simply have information on gangs and drugs that police want.

"Police say, 'Give me guns and I'll let you go,' or, 'Give me drugs and I'll let you go,'" Hill said. Reports have emerged of police abusing and even torturing arrestees until they agree to talk. One man left Homan Square with head injuries, while another mysteriously died in the interrogation room.

The Chicago police department's interrogation techniques reportedly include prolonged shackling, family threats, and demands to implicate others. In fact, many of them inspired the practices used at Guantanamo Bay, according to another Guardian investigation into Chicago's police brutality.

We have reached out to the Chicago Police Department and will update if we hear back.