Usually, the first element needed before a murder case can be brought is a body. In an exception to that rule, the U.S. Attorney's Office here charged a man last week with murder in a case where the body has yet to be found. What's more, another man has already pleaded guilty to murder in the case.

The victim was Patsy Gaisior of Harrisburg, Pa., a telephone company employe who was kidnaped Dec. 3, 1980, in Harrisburg and brought here. Her body has never been found.

Robert A. Ruff, 22, of Philadelphia, and Frank Johnson, also 22, of Harrisburg, were convicted last year of kidnaping Gaisior. Both men were sentenced last February to life in prison and are now in jail.

Last week, prosecutors charged Ruff with felony murder in the case. Johnson pleaded guilty last month to second-degree murder before D.C. Superior Court Chief Judge H. Carl Moultrie I.

Steven D. Gordon, an assistant U.S. attorney in charge of felony cases, said in court that Gaisior had been raped, killed, and thrown into the Anacostia River.

Gordon informed the judge that Johnson was cooperating with prosecutors in the investigation. In return, Gordon said, Johnson has agreed to tell the judge of the degree of his cooperation when he is sentenced. Gordon declined to provide further details, saying the case was still being investigated.