A man has been charged with practicing without a license after police say he collected large quantities of blood from D.C. residents and paid them for it.

Khoa Hoang Nguyen, 43, of Rockville, Maryland, was arrested for allegedly collecting blood from people in an abandoned apartment on P Street SW, police said.

The people were paid $30 each for the blood samples, according to charging documents.

"It was free money," said a woman who lined up but said she didn't have a chance to sell her blood.

Officers who were called to the scene Wednesday found a large group of people standing outside the building. Neighbors told News4's Darcy Spencer they had been seeing long lines of people going into the apartment.

"It was a long, long line. We didn't know what was going on," one neighbor said. "People were getting blood taken from their arm, and they put this tube stick down their nose."

A woman inside the unit where the blood was being drawn told officers she was part of a "work study" and the unit was the "source location," charging documents state. She told police Nguyen was in charge of the work study and called him over from across the street.

Nguyen told police he worked for Boston Biosource, a Newton, Massachusetts-based company that says it provides blood and tissue samples to clients, according to its website.

Nguyen said he was approved and certified to draw blood and that he had drawn blood from 40 people between Tuesday and Wednesday, but a ledger police found in the apartment listed an additional 205 names, according to charging documents.

Police said they also found large quantities of blood and used needles in the apartment. Police said the blood was stored improperly.

Nguyen was not able to provide any documentation, licenses or certification to prove he was qualified to draw blood. Police say the collected blood was not being stored in a manner consistent with "professional practices."

Police said they were not able to get in touch with a representative of Boston Biosource.

Boston Biosource told News4 Nguyen goes to risky areas to collect blood for research, looking for cures to diseases like cancer, tuberculosis and Alzheimer's disease, calling it valuable work.

Nguyen has been charged with practicing registered nursing without a license.