Bernard L. Madoff has maintained for years that his decadeslong Ponzi scheme was a one-man show. On Monday, a jury concluded instead that it was a team effort.

Jurors found five former employees of Mr. Madoff guilty of aiding and hiding the fraud in a trial that painted the money manager's Manhattan offices as a hive of illegal activity, where employees cooked up lies and manufactured fake documents to keep afloat a scam that ultimately cost investors $17 billion.

Computer programmers Jerome O'Hara, 51 years old, and George Perez, 48, were convicted in federal court in Manhattan of creating phony customer accounts, while portfolio managers Annette Bongiorno, 66, and JoAnn Crupi, 53, were convicted of concocting phony trading records. Daniel Bonventre, 67, a former operations director for Mr. Madoff, helped gin up false books and records, the jury found.

The findings reshape the narrative from a crime that has been tightly linked in the public's consciousness with one man to a group effort that transpired within the firm's offices in the iconic Lipstick building in midtown Manhattan.

Mr. Madoff directed the scheme from his office on the 19th floor, prosecutors told jurors, but much of the work done to conceal it took place in the secretive offices of the 17th floor, where the investment-advisory business was run. On that floor, dubbed "House 17" and accessible only with a security card, Messrs. O'Hara and Perez created computer programs to randomly generate false documents, while Ms. Bongiorno and Ms. Crupi backdated nonexistent trades, prosecutors said.