See comments and links to other NYP articles that explain what exactly was done:
http://www.freerepublic.com/focus/f-bloggers/2147748/posts - I Knew Bernie Madoff Was Cheating—That’s Why I Invested with Him
The traders worked for the market-maker, not the asset manager. By all reports, there was basically no contact between the two. One guy posting at Dealbreaker Friday said he worked for the market-making operation for a few years, and never met a single person who worked in the legendary asset management business, even though it was supposedly housed in the same building. It actually IS possible that Bernie was running it all himself. I won’t be surprised if it turns out one or both of the sons were in on it, but it is by no means impossible for one man with Bernie’s expertise and connections to have pulled this off alone, especially given that he obviously had unfettered access to the infrastructure of the broker-dealer/market-maker.