Bernard Madoff, who is serving a 150-year sentence in North Carolina for running a fraud scheme that cost investors billions of dollars, was physically assaulted by another inmate in December, according to three people familiar with the matter.

After the attack, Mr. Madoff, who pleaded guilty a year ago and was sent to a federal prison in Butner, N.C., was moved on Dec. 18 to the prison's low-security medical center for treatment. At the time, the Bureau of Prisons said that rumors of an assault were false and that Mr. Madoff suffered from dizziness and hypertension.

One of his lawyers, Ira Sorkin, added at the time that Mr. Madoff was experiencing high blood pressure and heart palpitations. Mr. Sorkin declined to comment Wednesday on whether his client was beaten, saying, "I don't comment on prison conditions or his family. That has been my policy."

Mr. Madoff was treated for a broken nose, fractured ribs and cuts to his head and face, according to a felon currently at Butner serving time on drug charges who was familiar with his condition at the time. The details of the injuries couldn't be independently verified.

Another inmate who recently was released from Butner after serving time on drug charges also confirmed the assault, as did a third person familiar with Mr. Madoff's situation.

The former inmate said the dispute centered on money the assailant thought he was owed by Mr. Madoff.

The current inmate said Mr. Madoff's assailant was a beefy man serving time for a drug conviction. The alleged assailant's mother said in an interview her son had not mentioned any scuffle with Mr. Madoff but that he had been a body builder and held a black belt in Judo until he was injured in a shooting in 2002. While behind bars he has regained strength and gotten back into shape, she said.

The Bureau of Prisons said it investigated the incident, a process that included an interview with Mr. Madoff.

"In December he told staff he was not assaulted, and an investigation was completed following his statements, which corroborated his statements," said Traci Billingsley, a Bureau of Prisons spokeswoman. "Not one inmate has told staff he was assaulted."

When The Wall Street Journal asked about the alleged assailant, she said the scenario would be "virtually impossible" because that prisoner lived in a different unit from Mr. Madoff, and each unit was locked down at night.

Denise Simmons, a spokeswoman at the Butner prison, said, "We have no knowledge or information to confirm he was assaulted."

It's not uncommon for prisoners to deny being beaten because they don't want to risk a reputation as a snitch, according to prison experts and prisoner advocates.

High-profile inmates may be at additional risk because other inmates may assume they have money or access to other resources, advocates say.