Washington D.C. - Mario Molina was crossing the Ysleta Bridge into Mexico from El Paso, Texas, last May when he was stopped by a Customs and Border Protection officer. Molina, an American citizen, said the CBP officer asked him where he was going and how much money he had.

About $170 or $190, Molina thought at the time. When he reached into his wallet to check, the officer allegedly said: "That's it - you're under arrest. I'm taking you in for questioning." Molina asked what probable cause he had, and he said the officer responded that he didn't need one.

Then, according to Molina, he was taken into an inspection room where officers asked him about his hair color, eye color and skin color. When Molina said the answers were apparent, an officer allegedly slapped him across the face, saying, "You better stop playing your (expletive) silly games with me and do what I tell you to."

After nearly three hours, Molina was released back into El Paso and his belongings were returned, he said. But he noticed that a few things were missing: a silver ring, and nearly $200 in cash.

Molina's story is among 13 allegations detailed in a complaint filed by the American Civil Liberties Union and Southern Border Communities Coalition on Tuesday.

In a letter to the U.S. Department of Homeland Security and Customs and Border Protection, the advocacy groups call for an investigation into "unchecked abuse" at the southwest U.S.-Mexico border.

The Southern Border Communities Coalition is a collection of more than 60 organizations doing human rights work at the border.

"This complaint includes multiple individual complaints of abuse at Southwest border (Port of Entries) involving excessive force; the use of coercion to force individuals to surrender their legal rights and citizenship documents; and the lack of a clear, transparent, complaint process for individuals to seek redress," the letter said, noting that the accounts reflect "broader patterns" of misconduct.

DHS has not commented on the complaint. At the close of an internal investigation into employee behavior in the agency, a March statement on the CBP website noted that the agency is moving "toward a more transparent and accountable approach to enforcement and workforce integrity, while protecting the public and the men and women of CBP."

In a statement to Buzzfeed News, the Customs and Border Protection agency said: "CBP does not tolerate discrimination nor mistreatment and takes complaints, to include the allegations made in the May 17, 2016 letter, seriously."

Those who claim to have been unfairly detained and abused include American citizens, permanent residents and Mexican citizens with valid visas for visiting the U.S.