The U.S. policy of sending some migrants back to Mexico as their asylum cases proceed has led to hundreds of cases of violence, according to a recently updated report.

Through news stories, policy reports and its own on-the-ground research, the advocacy group Human Rights First has documented more than 600 cases of kidnapping, torture and sexual assault against such asylum seekers.

Tens of thousands have been returned through a program called the Migrant Protection Protocols, or MPP, which was rolled out by the Trump administration in late 2018.

“Many people who are seeking asylum, with legitimate, valid asylum claims, are being returned to Mexico to wait in this horrendous danger,” said Kennji Kizuka, a senior researcher with the advocacy group.

He said asylum seekers face danger even in border migrant shelters, which his group’s researchers found rarely have police protection. Because the report was based only on incidents documented by reporters, lawyers or advocates, there are likely many more unreported instances of violence, according to Kizuka.

In a Thursday release announcing MPP’s implementation in Nogales, Customs and Border Protection said the agency “strives to maintain a safe and orderly flow of trade and travel at the ports, including the processing of claims for asylum.”

CBP said that on Wednesday nine Venezuelan nationals tried to cross through the vehicle lanes without proper documentation.

“They were returned to Mexico through the Port of El Paso to await the next steps in the immigration process,” the release reads.

Asked in mid-November about reports of violence faced by returned asylum seekers, Acting Customs and Border Protection Commissioner Mark Morgan described such claims as anecdotal. He also said the program has decreased overcrowding at agency facilities.