In the Midwestern heartland, police are encountering a new social evil: trafficking, often involving women and children who are forced to work as prostitutes or unpaid labour; and the outcomes can be brutal.

Human trafficking has become a major issue in the Midwest heartland of America, causing some campaigners to dub it a modern form of slavery.

Figures from the State Department reveal that 17,500 people are trafficked into the US every year against their will or under false pretences, mainly to be used for sex or forced labour. Experts believe that, when cases of internal trafficking are added, the total number of victims could be up to five times larger. And increasing numbers of trafficked individuals are being transported thousands of miles from America's coasts and into heartland states such as Ohio and Michigan.

"It is not only a crime. It is an abomination," said Professor Mark Ensalaco, a political scientist at the University of Dayton, Ohio, who organised a recent conference on the issue. In Ohio a human trafficking commission has just been set up to study the problem, while in the northern Ohio city of Toledo a special FBI task force is tackling the issue. For many local law enforcement officials, it is a bewildering new world.

In one recent incident a 16-year-old Mexican girl was found to have been trafficked across the US border. Doctors noticed the heavily pregnant girl showed clear signs of physical abuse when she was brought into a hospital in Dayton to give birth. The police were called but the couple who had brought her had already fled. When the girl's story emerged, it became clear she had been kept against her will in the nearby city of Springfield and used for labour and sex. "I thought slavery ended a few centuries ago. But here it is alive and well," said Springfield's sheriff, Gene Kelly.

He emphasised the risks to the girl's baby after it had been born if the doctors had not been so alert: "Like the mother, the baby could have ended up a victim for years to come. Who knows? Future labour? Future person to traffic?"

Ohio anti-trafficking campaigner Phil Cenedella, founder of Combating Trafficking Anywhere, believes that the baby was destined to be sold off by her captors. "They would have put the kid on the black market. It is crazy that this is happening." Human trafficking – defined as forcing someone against their will to work for no reward – has been dubbed modern slavery. At the Dayton conference, it was discussed as a growing social problem, not in some far-off foreign land, but among the cornfields of Ohio.

"The problems are broader than we realised," said Ohio's attorney general, Richard Cordray. "What we want to do is find and disrupt these networks."

One of the country's leading anti-trafficking advocates is Theresa Flores, a former victim. Flores puts a different kind of face on human trafficking in America. She is white, middle-class and blond and looks the epitome of a suburban American woman. She grew up in a wealthy suburb of Detroit in Michigan and did well at school. Yet Flores tells a nightmarish story of two years being drugged, raped and sold for sex.

Flores, whose ordeal was turned into a book called The Sacred Bath: An American Teen's Story of Modern Day Slavery, was attacked and raped when she was 15. Her assailant used the threat of photographs he had taken during her rape to force her into having sex with strangers. She became the effective prisoner of a drugs gang that used her as a prostitute and kept her earnings, or gave her away free to gang members as a "reward". "People don't think that trafficking looks like me or that it can happen to someone who came from a nice neighbourhood. But it does. People need to see outside that box," said Flores.

Flores said that her lowest point came when the gang took her to a seedy motel where she was raped by as many as two dozen men. She woke up alone, abused and with no clothes. "I was told I would die if I told anyone. It happened over and over for two years as I became a sex slave for those men," she said.

Anti-trafficking campaigners point out that cases in the US come in a wide variety of forms involving men, women and children. One major area is that of trafficked labour with people used for domestic work or, more commonly, for back-breaking labour in agricultural industries. But trafficking cases have also occurred in businesses such as restaurants, hair salons and beauty parlours. The overwhelming majority of the rest are sex cases, usually involving young women or children forced into prostitution. The methods used to keep people vary. They include confiscating the passports of those brought in from a foreign country or the threat of extreme violence. Other tactics are to threaten family members if a victim does not comply or, as in Flores's case, to use blackmail.

Trafficking represents a new challenge to law enforcement, especially in regions which have traditionally not thought of it as a major problem. That is especially true where it happens within an immigrant community. Languages are a problem as well as cultural issues and a natural fear that many immigrants – some of them possibly illegal – have of contacting the police.

Kelly believes that is the case in Springfield, a town that is almost the Midwestern archetype. It was once featured in a story in Newsweek magazine entitled "The American Dream". But its 65,000 citizens also face all the problems of a modern America in the grip of a deep recession: an immigration crisis and profoundly changing demographics. The town now hosts several prominent minority communities who make up more than a fifth of its population, including Russians, Chinese, Latinos and Somalis. "There are a lot of people who distrust law enforcement. We need to break down those barriers. Our officers need training, especially in languages," said Kelly. "If you can't speak to people, you can't reach them."

Some commentators and experts have accused victims' advocates and academics of overstating the problem, arguing the problem has been exaggerated and expressing scepticism at the notion that vast organised criminal networks are dealing in human beings for sex or labour. Law enforcement officers also acknowledge that the definitions of trafficking may need refining.

In North Carolina last week the mother of a five-year-old girl was charged with human trafficking after being accused of offering her daughter for sex. The child was later found dead. The crime was horrific, but the distinction between trafficking and simple, sadistic child abuse might not be immediately obvious.

"We have a problem with definition. It is not always straightforward and easy to explain," said Laura Clemmens, a government lawyer in Dayton. "The hard part is bringing it into the light. At the moment these crimes are clouded in secrecy."