A former WorldCom finance chief, Scott Sullivan, was sentenced to five years in prison – 22 years less than the maximum term – for helping his boss, Bernard Ebbers, lead an $11 billion fraud that ended in bankruptcy.

Sullivan, 43, was Ebbers’s top deputy and later became the star witness against him. The reduced sentence, which includes three years’ probation, was a reward for cooperating with the government and testifying against Ebbers. Prosecutors said Sullivan was “perhaps the key factor” in convicting Ebbers, WorldCom’s former chief executive officer, of conspiracy and fraud. Ebbers was sentenced last month to 25 years in prison.

“I deeply regret my actions, and I sincerely apologize for the harm they have caused,” Sullivan told U.S. District Judge Barbara Jones in Manhattan federal court this morning. “I made horrible decisions. I did not stand up as the person I am.” Judge Jones called Sullivan “the architect of the fraud at WorldCom” and said his crimes “were of the highest magnitude.” She accepted the government’s recommendation for leniency as a reward for Sullivan’s cooperation. “He was an excellent witness,” she said.

The judge said she took into account that Sullivan’s father has Parkinson’s disease, and that his wife, who’s suffering from diabetes, will need to care for the couple’s 4-year-old adopted daughter. Judge Jones said she reduced Sullivan’s sentence accordingly. Under sentencing guidelines, he could have received a term ranging from almost 22 years in prison to a maximum of 27 years and three months. Over seven days on the witness stand in February, Sullivan told jurors he repeatedly warned Ebbers that WorldCom accountants were making false entries so the company could hit its financial targets. Ebbers refused to inform investors about the company’s slowing growth, Sullivan said.

WorldCom, the no. 2 American long-distance company, collapsed in 2002 after disclosing that it inflated revenue and hid expenses to meet expectations.