In just 15 days, Kenwood Allen killed four people, prosecutors with the Manhattan district attorney’s office said on Wednesday.

Mr. Allen, 33, was already charged late last year after prosecutors said he was part of a criminal operation that targeted people emerging from bars on Manhattan’s Lower East Side after a long night out. Prosecutors said that he would drug victims with fentanyl and other opioids before stealing their credit cards and other valuables, often leaving them unconscious on the street.

On Wednesday, the Manhattan district attorney, Alvin L. Bragg, said that further investigation had revealed the full breadth of Mr. Allen’s criminal operation. He is now accused of targeting 21 victims between March and December of 2022, and killing five people within five months, including the four between July 22 and Aug 6.

Mr. Allen has been charged with 10 counts of second-degree murder — two for each death, one accusing him of acting with depraved indifference to human life, the other of acting in furtherance of another crime. He was also charged with 17 counts of robbery and attempted robbery. He pleaded not guilty at his arraignment on Wednesday.

The Legal Aid Society, which is representing Mr. Allen, did not respond to a request for comment on the charges.

A prosecutor with the office, Brian Rodkey, said that Mr. Allen, who is also charged with conspiracy, worked with at least one other person who is expected to be charged as his co-defendant.

At first, authorities believed that the overdose deaths in downtown Manhattan throughout 2022 were unrelated. But by April, the police and prosecutors with the district attorney’s office had uncovered not one but two criminal operations in which victims were drugged after they emerged from bars, with fatal consequences.

The case against Mr. Allen is separate from that against five other men who were accused in April in a series of druggings and killings. That group, prosecutors said, targeted the patrons of gay bars, raising fears in New York City’s L.G.B.T.Q. community. A trial date has not been set in that case.

“These alleged pernicious drugging and robbery schemes have left far too many families mourning the loss of their loved ones,” Mr. Bragg said in a statement.

Mr. Allen was sent to jail after his initial indictment last year, and after a brief appearance in a Manhattan courtroom on Wednesday, the judge in his case, Felicia A. Mennin, said he would be sent back.