On March 23, when Willian Guillen's shift at the Peruvian restaurant Millenium Chicken on New York's Long Island ended around midnight, he and co-chef Neptali Robles walked to a nearby restaurant for post-work takeout food. While they waited for their order of papusas, they stopped to say hello to a friend at another restaurant a block away and dashed into a 7-Eleven for an errand.

Suddenly, two men came running and yelling from a dark street corner about a block from the restaurant. Guillen, a native of El Salvador, had worked as a chef in the U.S. for more than a decade, but he understands little English. Thinking he was about to be robbed, he ran back toward the restaurant where he had ordered his food, screaming to a group of men outside to call the police.

The yelling men, as well as those in front of the restaurant, were members of an undercover unit of the Nassau County Police Department. According to a complaint filed with federal officials, the officers tackled Guillen, handcuffed him and beat him severely. They dragged him across the pavement in front of the restaurant and repeatedly stomped his head, according to the complaint.

Guillen's ordeal did not end there, according to his attorney, Karen Bobley. In her complaint seeking a federal investigation of police conduct in the case, she says Guillen was taken to a police holding facility, where officers beat him again. Guillen, an undocumented immigrant, was later transferred to the Nassau County Correctional Center, where he has been locked for nearly eight months awaiting possible deportation.

“My client was not only beaten, he is completely innocent,” Bobley told The Huffington Post. “He continues to suffer from the physical effects of the beating and has had completely inadequate medical care. He has been utterly disregarded by the DA's office, who failed to indict, yet continue to pursue a disingenuous prosecution despite overwhelming evidence of his innocence.”

Police alleged Guillen and Robles had engaged in a drug transaction, and said they saw Guillen throw a plastic bag of cocaine into the street as he ran. Surveillance video from outside the restaurant does not show a drug deal, and no cocaine was recovered from the scene, according to Bobley. Guillen was charged with assaulting a police officer, tampering with physical evidence and resisting arrest. Robles was charged with resisting arrest and misdemeanor drug possession.

The Nassau County Police Department did not respond to requests for comment.

“In this case, the defense attorney asked our office to investigate allegations of police misconduct, which we did immediately,” said Paul Leonard, press secretary for Nassau County District Attorney Kathleen Rice, denying that the office has in any way disregarded Guillen or his case. “The DA’s office has been thoroughly investigating these allegations and we will let only the facts dictate our course of action in this case.”

Robles was released on bail the day after his arrest and the charges against him were eventually dismissed.

According to Bobley's complaint, the officers at the holding station forced Guillen to undress, then beat him mercilessly. The officers laughed at Guillen, called him names and tried to kick him in his genitals as he lay on the floor, the complaint says. They stomped his thigh so forcefully that the bone was exposed, according to the complaint.

Guillen was hospitalized for five days, then transferred to the correctional facility in Nassau County. Three of his ribs were broken. His face was swollen, and bruises and abrasions covered his head, back, knees, chest and elbows. Toxicology tests were negative for all substances, including alcohol. Guillen has no criminal history.

Bobley complained about Guillen's treatment in Oct. 19 letters to New York Attorney General Eric Schneiderman, several assistant U.S. attorneys, the Department of Justice, the FBI, and President Barack Obama. She sent a copy to U.S. Attorney Loretta Lynch, the president's nominee to replace U.S. Attorney General Eric Holder.

Bobley said she had received a response from Lynch’s staff and plans to meet with the office this week.

Bobley has met with the El Salvadoran consulate and was told an official would ask U.S. immigration authorities not to take action on Guillen’s deportation.

After Guillen had been jailed for six months, Bobley said the Nassau County DA's office offered to drop the charges if he would plead guilty to disorderly conduct. "Willian rejected that offer and insists that he will never plead guilty to anything," Bobley said.

Bobley has filed a formal motion in court to dismiss the charges. She said she hopes the Justice Department investigates Guillen’s beating, as it did recently in the case of a man beaten by police in California.

If Guillen is certified as a material witness in a federal investigation, he could apply for a U visa, a special status granted to immigrants who are victims of a crime and cooperating with law enforcement.

"The people that did this to me ... they don't have the heart of a human being,” Bobley said Guillen told her.