A handwritten, signed account of how the 1984 murder of Alexandria pediatrician Robert S. Rixse was purportedly planned and executed has provided police a major break in the investigation and suggests that several persons were involved in the slaying, according to sources.

The account, a copy of which was found last month in a police search in Marietta, Ga., allegedly was signed with the name of a former Alexandria resident, John Kennedy Robinson, sources said. Robinson, 51, has been charged by Alexandria police with conspiracy to commit murder in connection with Rixse's death, a charge that his attorney denies.

Alexandria police concluded early on that Rixse was slain by a professional gunman, they have said. Further, Alexandria police have long been investigating whether Diane (Mimi) Robinson is involved in the slaying, according to sources. She separated from Rixse in a bitter 1981 divorce and married John Robinson the next year.

Reached recently at her Marietta home, where she moved after Rixse's murder, Diane Robinson declined to be interviewed.

Diane and John Robinson were divorced in May after separating last October, according to papers filed in the Cobb County, Ga., courthouse.

A copy of the purported handwritten account of Rixse's murder was found in a police search of a business office once used by John Robinson in Marietta, the sources said. Sources said the account said a copy should be sent to Alexandria police investigators.

John Robinson's attorney, Garland T. Byrd of Butler, Ga., said he was told by Alexandria authorities that such an account exists but that he had not seen it.

"It was written at a distressful moment; he was under pressure" from his failing marriage to Diane Robinson, Byrd said.

Robinson is being held in Pensacola, Fla., pending a trial next month on apparently unrelated federal drug charges. He has pleaded not guilty to those charges, Byrd said. Federal authorities, who arrested him Aug. 5 on the drug charges, said that on Aug. 21 they received a detainer charging Robinson with conspiracy to commit murder.

The authorities said that Robinson will not be brought to Alexandria until after his Oct. 19 federal trial. Byrd said his client denies the murder conspiracy charges and "is willing to come {to Alexandria} at any time."

Sometime after the murder, Robinson told The Washington Post that he and his wife were "totally baffled" by the slaying and knew nothing about it.

Alexandria police decline to comment on the Rixse murder investigation, which they say is continuing.

Rixse, 37, was shot in the head with a handgun on July 1, 1984, when he opened the front door of his home at 315 Kentucky Ave., in the Beverley Hills section of Alexandria. His three young daughters were sleeping upstairs.

His slaying shocked Alexandria, where Rixse was a popular doctor and community activist. His murder followed the bitterly contested 1981 divorce, after which Rixse obtained custody of the three children. At the time of the murder, Rixse was involved in an acrimonious court battle with his former wife over division of some of their joint property.

Soon after the shooting, the Robinsons sold their Alexandria house at 812 Chalfonte Dr. for $225,000 and moved to Marietta, a suburb of Atlanta. There, John Robinson, a native Georgian, got a job as a lobbyist with the Independent Insurance Agents of Georgia and Diane Robinson began working as a real estate agent after obtaining her license in November 1984.

In August of 1984, Diane Robinson purchased a two-story house in the Marietta subdivision of Indian Hills, according to a deed on file in local court. The house, surrounded by tall pines and located near a golf course, was valued a year later at $135,000, according to court papers.

According to neighbors in Marietta, the Robinsons were a socially active, gregarious couple who entertained frequently. On one occasion in the fall of 1984, the Robinsons held an open house for Rep. George (Buddy) Darden (D-Ga.) when he was running for reelection.

"It was an absolute fiasco," Darden said yesterday of the open house, at which only about five people showed up. The congressman said that he did not know John Robinson but had agreed to the reception at the request of a close friend who knew Robinson.

A year later, Robinson was arrested and charged with felony theft, according to papers filed in Cobb County Superior Court in Marietta. An indictment handed up in January 1986 charged that he had placed $7,350 of his employer's funds into his own bank account instead of depositing it, as instructed, into a high-interest-bearing account held by the insurance agents' group.

He pleaded guilty in August 1986 to that charge. He was given probation under Georgia's first-offender statute, fined $1,000 and ordered to pay $10,800 in restitution, court papers show.

A few months earlier, the Robinsons had set up a property management and rental company jointly with neighbors Dale and Robert Mehlhouse, according to court papers. John Robinson was listed as the registered agent for "Home Management Associates Inc." Diane Robinson and the Mehlhouses were listed as officers of the company.

The business at first was run out of the Robinsons' home, but sometime this year it was moved to the Mehlhouses' home next door, sources said. It was in this basement office where police last month found a copy of the purported account of the Rixse murder, sources said.

Cobb County Magistrate James G. Bodiford declined to reveal what was confiscated under the search warrant, which he said was returned Aug. 27.

In October of last year, the Robinsons separated and Diane Robinson requested a divorce, according to court papers. The couple signed a divorce agreement Nov. 21, 1986, but did not file it in court until March, according to court records. The divorce became final in May, the records show.

The divorce greatly upset John Robinson, an associate said. "It almost destroyed John," he said.

Under the terms of the divorce settlement, Diane Robinson retains sole custody of her three children. Her former husband, who legally adopted the girls after their father's death, has visiting privileges.

Diane Robinson has failed in Virginia and Georgia courts to overturn an August 1984 order by Alexandria Circuit Court Judge Donald H. Kent granting visitation rights with Diane Robinson's children to Rixse's mother, who lives in Alexandria.

Kent later authorized Diane Robinson's arrest if she ever returned to Alexandria so she could be brought to court to see whether she had failed to comply with his 1984 order on the visitation rights.