Reports of domestic human trafficking continued to increase in 2016, jumping 35 percent over 2015, according to a report released on Tuesday by the National Human Trafficking Resource Center and the Polaris Project.

Human trafficking is the act of forcing someone into labor -- including sex work -- against their will. Due to the hidden nature of the crime, authorities and academics alike struggle to determine the scope of the problem. One source of quantifiable information about trafficking in the U.S. is a hotline set up in 2007 by Polaris, an anti-trafficking organization.

Calls to the hotline and texts to the BeFree textline led to the opening of over 8,000 cases in 2016, up from 5,961 cases in 2015 and 5,382 cases in 2014. Almost three-quarters of the calls were made in regards to sex trafficking, with labor trafficking making up just 14 percent of the hotline's call volume in 2016.

The report also analyzed available information on how trafficking victims were initially exploited by traffickers. Intimate partners and family members of victims were the traffickers in more than half the sex trafficking cases the report analyzed. For labor trafficking victims, more than half were exploited with the promise of a job offer. The top risk factors for victims were "recent migration/relocation" and "substance use concerns," the report said. Traffickers forced their victims to continue working for them through emotional, economic and physical abuse, as well as isolation.

Reported incidents of human trafficking have climbed over the last decade, but it's unclear whether that increase is a reflection of a growing prevalence of the crime or a growing awareness of it. Similarly, it's unclear whether the rise of an online sex market over the last twenty years has increased the sex trade, or just made it more visible.

In previous decades, sex work was most likely to be assumed to be a "victimless crime" and the sex worker in question was assumed to be doing the work out of his or her own free will. But that perception has changed, thanks in large part to work by activist groups, and now a sex worker is more likely to be seen as a trafficking victim both in the eyes of the public and the law.

Anti-trafficking efforts were embraced by evangelical Christians in the late 1990s. The first federal human trafficking law, the Trafficking Victims Protection Act (TVPA), was passed in 2000, and reauthorized in 2005, 2008 and 2013.

In 2003, the FBI started its Innocence Lost National Initiative and began launching nationwide stings of human traffickers. In 2007, the Justice Department launched a Human Trafficking Prosecution Unit.

Increased interest in the crime by law enforcement has led to an understanding among advocates that victims of sex trafficking are often trapped in sex work by criminal charges for prostitution and drug possession. In September, Sen. Kirsten Gillibrand (D-NY) introduced a bill that would clear the records of human trafficking victims, who are often arrested on prostitution and drug charges while being trafficked.

In November, the disappearance of California mom Sherri Papini highlighted the threat of sex trafficking. Papini was missing for three weeks before she was found, and authorities said three other women in the county were missing as of the beginning of January.