An independent investigation has found that imprisoned former Rep. Randy “Duke” Cunningham took advantage of secrecy and badgered congressional aides to help slip items into classified bills that would benefit him and his associates.

The finding comes from Michael Stern, an outside investigator hired by the House Intelligence Committee to look into how Cunningham was able to carry out the scheme. Stern is working with the committee to fix vulnerabilities in the way top-secret legislation is written, said congressional officials who spoke on condition of anonymity because the committee still is being briefed on Stern’s findings.

Cunningham’s case has put a stark spotlight on the oversight of classified – or “black” – budgets. Unlike legislation dealing with social and economic issues, intelligence bills and parts of defense bills are written in private, in the name of national security. That means it is up to members of Congress and select aides with security clearances to ensure that legislation is appropriate.

Federal prosecutors found that Cunningham accepted $2.4 million in bribes, including payments for a mansion, a Rolls-Royce and a 65-foot yacht, in return for steering defense and intelligence contracts to certain companies. Cunningham pleaded guilty and was sentenced to more than eight years in prison.

Rep. Peter Hoekstra, R-Mich., said he still has questions about how much Cunningham relied on legislation and how much he bullied people at the Pentagon to direct money to certain contractors.