NEW YORK (Thomson Reuters Foundation) - Arresting survivors of human trafficking for their testimony in criminal prosecutions creates trauma and trouble returning to society, research by a leading U.S. trafficking group showed on Thursday, calling for an end to the practice.

Survivors detained as material witnesses to help prosecute their traffickers can lose custody of their children, jobs and services they need for recovery, said the Human Trafficking Legal Center (HTLC), a legal group for trafficking survivors.

By law, witnesses who may not want to cooperate can be detained if they have testimony needed to prosecute a criminal case.

The number of trafficking survivors held under material witness warrants is uncertain, as such warrants are typically used in secrecy, the HTLC said.

The U.S. Department of Justice initiated more than 200 human trafficking prosecutions and convicted more than 500 people, according to the latest data from 2018.

The detention practice makes criminal justice a priority over victims’ needs and wants, said the HTLC which examined 49 cases of victim arrests between 2009 and 2018.

“They’ve experienced trauma of being locked up and being forced to do things they didn’t want to do,” said Martina Vandenberg, HTLC president.

“In a sense it replicates much of the trauma they’ve already experienced,” she told the Thomson Reuters Foundation.

Representatives of the Justice Department did not immediately respond to requests for comment or an estimate of the numbers of survivors held.

“While material witness warrants are sometimes necessary in federal criminal cases, they should be an absolute last resort in a trafficking case,” said Luis C.deBaca, former U.S. anti-trafficking ambassador-at-large.

“In a trafficking case there is a cruel irony in using this type of coercion as a instrument to punish a trafficker who held that same person through coercive means.”

The HTLC also said the pandemic provided more reason to release trafficking victims, as concerns mount over the coronavirus spreading through crowded inmate populations.

The pandemic has prompted inmate releases, reduced bail requirements and other measures to reduce jail populations around the country.

An estimated 400,000 people are believed to be trapped in modern slavery in the United States, according to the Global Slavery Index which is published by the human rights group Walk Free Foundation.