The investigation also found cases of the bail bond system being used in sex trafficking operations in at least five different states. Pimps and sex buyers are locating incarcerated women awaiting a court date by using personal data such as mugshots and bail bonds posted online, or through corrupt bondsmen.

Traffickers are then bailing women out of detention. Once released, the women are told they must work as prostitutes or have their bond rescinded and be sent back to jail.

Over the course of the investigation, The Guardian found cases of the bail bond system being used by pimps and sex buyers in Florida, Texas, Ohio, North Carolina and Mississippi.

“The pimps would use bail as a way to control us and keep us in debt bondage,” said one trafficking survivor from Tampa, Florida. She claimed she was forced to work as a prostitute to pay off her bail debt and locked inside a house and beaten if she didn’t bring home enough money.

“Once when I tried to escape, the pimp revoked my bond. He found me, threw me in a car and got me sent back to jail,” she said.

Diane Checchio, a former prosecutor for the district attorney’s office in Orlando, Florida, said the bail bond system was routinely exploited by traffickers.

Up to 80 per cent of the trafficking cases she worked on in 2016 involved bondsmen found to be illegally passing on information about women arrested on prostitution charges to suspected traffickers.

“Sometimes women are released not knowing who bonded them out or why, or what they’ve got into, and now they’re being coerced,” Checchio said. “They come out of jail and there’s someone waiting saying: ‘I posted your bond – now you owe me’ … [They will] threaten to rescind that bond if the girls don’t do what they’re asked or told to do. It’s still happening now.”

Checchio said traffickers were likely to be targeting women involved in the criminal justice system across the country.

“I would find it very likely that this is happening in every state that has women’s records online,” she said.

Once they have identified a potential target inside a prison or jail, traffickers will try to establish a relationship by using letters, phone calls and promises of money and housing when the victim is released. Prison bank accounts are also used to send money to women, establishing a debt that is used to coerce them into prostitution on their release.

“When I was in prison [in Ohio] I had pimps I knew from the streets, and men I had never met, writing to me to try and convince me to go home with them,” said Amy Williams*, who was incarcerated in state prisons and county jails in Ohio over a 15-year period.

“Some of us knew what we were going back into but didn’t feel we had any other choice, as they’d be waiting for us anyway. Other girls I knew had no idea that they would be put on the streets by this person.”

Pimp-controlled prostitution is now recognised as one of the most brutal and pervasive forms of human trafficking in the US. Trafficking is defined under US federal and international law as when a person is induced to perform labour or a commercial sex act through force, fraud or coercion.

Over the course of the investigation, The Guardian gathered testimony from more than 20 trafficking survivors in 11 states across the country, as well as correctional officers, convicted sex traffickers, law enforcement officials, lawyers, prosecutors and frontline workers. All corroborated that prisons and jails were being used as recruiting grounds for human traffickers.

There are currently 1.2 million women under the supervision of the criminal justice system in the US. Women now comprise a larger proportion of the prison population than ever before, with the number incarcerated eight times higher than in 1980.

Many women are jailed for non-violent offences, with 25 per cent incarcerated on drug-related charges. Many of the women interviewed had been jailed multiple times and had fallen in and out of the control of pimps over a period of years.

A 2017 survey of 130 trafficking survivors by the National Survivors Network, an advocacy and campaigning group, found that 91 per cent of respondents said they had been arrested not only for prostitution but also for the sale and possession of drugs and a range of other crimes.

“Predators thrive off isolation and trauma, so prisons and jails are perfect hunting grounds because there you have a captive population of women who often have nowhere to go, and no support when they’re released,” said Nicole Bell, a trafficking survivor and founder of Living in Freedom Together, an anti-trafficking organisation in Massachusetts.

“Now they have figured out how to work the system, these institutions have become like big fish bowls for traffickers. Incarceration takes vulnerable women and makes them more vulnerable,” she said.

Anti-trafficking campaigners said correctional facilities must do more to prioritise the safety and protection of inmates and ensure staff understand the vulnerability of their institutions to human trafficking.

“Our correctional facilities have a legal responsibility to protect the women who are under their charge,” said Marian Hatcher, a national anti-trafficking advocate and human trafficking coordinator at the Cook County sheriff’s office of public policy in Chicago.

“If inmates are being targeted while inside our prisons and jails by predators, instead of being offered the chance of an alternative when they are released, then this is a systemic failing of our duty of care to some of our country’s most vulnerable women.”

The US Department of Justice declined to comment.

*Name changed to protect identity