The job title is soigneur, an elegant sounding name for the person on a professional cycling team who is assigned some unglamorous work: massaging the muscles of the cyclists, laundering their clothes, booking their hotel rooms and preparing their food. Discretion and loyalty are also part of the job.

For Emma O’Reilly, a young, onetime electrician from Dublin, the chance in 1996 to be a soigneur for the United States Postal Service cycling team was an extraordinary opportunity. She had raced some as a teenager in Ireland, and served as an assistant on that country’s national cycling team. But the Postal Service team was a rising power, with its sights set on the Tour de France.

In short order, however, it became clear to Ms. O’Reilly that her tasks with the team would hardly be limited to kneading leg muscles and doing laundry. In an interview this week, Ms. O’Reilly said she became a regular player in the team’s doping program, one that investigators have charged took on its most sinister and far-reaching dimensions with the arrival of Lance Armstrong in 1998. Ms. O’Reilly, then not yet 30, said she wound up transporting doping material across borders, disposing of drugs and syringes when the authorities were lurking, and distributing performance-enhancing substances to the team’s riders whenever they needed them.

Discretion and loyalty, she said she came to understand, were not just valued qualities. They were paramount.