New evidence in the murder case against Dr. Jeffrey MacDonald has prompted the FBI to reopen the investigation at the request of a retired agent.

The evidence is a 53-page signed statement from Helena Stoeckley, who said she witnessed the murders of MacDonald's pregnant wife, Colette, 24, and two daughters by members of a satanic drug cult that broke into the MacDonalds' home at Fort Bragg, N.C., in 1970. That account supports MacDonald's version of what happened.

Stoeckley was interviewed by authorities before MacDonald's 1979 trial, but she gave confused and conflicting accounts of her whereabouts the night of the murders, and said she had been high on drugs.

After his conviction, MacDonald was freed when a federal appeals court ruled that his right to a speedy trial had been violated. The U.S. Supreme Court recently reinstated his conviction, and he is back at the federal prison on Terminal Island pending appeal on other grounds.

The ex-agent, Ted Gunderson, who was hired by MacDonald's defense team to investigate independently in 1980, said he is concerned about how aggressive the FBI investigation will be, noting that it has been more than a year since he informed the Justice Department of Stoeckley's statement.