A 43-year-old man has been charged with paying people $30 to draw samples of their blood, which D.C. police said he was storing in “large quantities” in an abandoned apartment near Nationals Park in Southwest Washington.

An arrest affidavit said police stumbled on the makeshift operation when a patrol officer saw people — many described in court documents as “habitual substance abusers” — congregating Wednesday afternoon in front of a three-story brick apartment building at P and First streets.

The officer overheard some of them discussing being paid to give blood, according to the affidavit filed by the Office of the D.C. Attorney General, and went to investigate. The suspect, Khoa Hoang Nguyen, told police that he was doing work for a biomedical company “and that he was approved and certified to withdraw blood from citizens,” according to court papers.

A woman at the apartment told police that she and Nguyen had drawn blood from 20 people that day, the documents state. They say that Nguyen had a ledger with an additional 205 names.

Authorities said they were unable to corroborate Nguyen’s account. The D.C. Health Department concluded that Nguyen had no medical license, according to the court documents. He was charged with one count of practicing registered nursing without a license. The woman was not charged and is listed as a witness in court documents.

Khoa Hoang Nguyen declined to comment after his hearing as he was leaving the court Thursday May 25, 2017. (Keith Alexander/TWP)

Nguyen, whose family came to the United States from Vietnam at the end of the Vietnam War in 1975, appeared Thursday for an initial appearance in D.C. Superior Court, where a judge ordered him released from custody. A hearing was set for June 23.

The judge ordered Nguyen, of Rockville, Md., not to perform any medical procedures that require a license while on release on the misdemeanor charge. Nguyen was also ordered not to draw any blood from anyone.

Nguyen declined to comment after the hearing. His court-appointed attorney, Lauckland Nicholas, also declined to speak on the specifics of the case. “We will just have to see what happens,” Nicholas said. “There is a presumption of innocence.”

It was not immediately clear what Nguyen intended to do with the blood. Police said officers also confiscated used needles from the apartment in the public-housing complex. Court papers and authorities did not say how much blood was found or how it was being stored.

Nguyen spent five years in prison after pleading guilty in 2010 to conspiracy to distribute drugs after an FBI raid on a doctor’s office he worked at in Falls Church, Va.

Authorities said that from 2005 through 2007, Nguyen assisted a licensed rheumatologist and, despite not having a medical license, prescribed a variety of powerful painkillers, including opiates, more than 3,600 times to 13 different patients. Prosecutors said neither Nguyen nor the doctor had performed any examinations.

The drugs, federal prosecutors said in court documents, included “some of the most addictive and heavily abused prescription drugs on the market.” Prosecutors wrote that Nguyen prescribed the medicine “without any training whatsoever” and wrote prescriptions to people “regardless of their medical condition.”

Nguyen’s family said in court papers that they had struggled to adapt to life in rural Pennsylvania, where they were located in 1975. One of his sisters went blind from a brain tumor, and his mother died in 2008. According to family letters sent to the judge, that devastated Nguyen and sent him into drinking spasms.

His father, two brothers, a sister, his girlfriend and others wrote heartfelt letters pleading for lenience from the federal judge. They described hardships moving a large family overseas to escape communism and having to leave their patriarch behind to finish his work with the U.S. government.

His then-girlfriend Patricia Meadows said in a letter to the judge that she and Nguyen had a young child. She said Nguyen had grown up wanting to be a doctor and that he had helped her mother and his mother through various sicknesses.

“Khoa has been working in the medical field in one capacity or another for the past decade,” Meadows wrote. “While working with patients, Khoa realized that he really enjoyed the challenge and satisfaction of being in the process of helping them get better. He told me he wished he had the opportunity to finish his medical degree.”

At the time of his federal conviction, Nguyen had been enrolled in the medical program at the University of Science Arts & Technology in the British West Indies.

The school’s dean of admissions, Orien L. Tulp, wrote the court that they would be willing to take him back as a student even with a felony conviction. “We value him as a worthy and hard working student with the ability to become a fine physician,” the letter said.

Editor’s note: An earlier version of this story said the incident happened Monday. It occurred Wednesday.