Federal and local investigators probing the mysterious disappearance last Aug. 31 of popular Washington area disc jockey Bob (Nighthawk) Terry now believe he was murdered, The Washington Post has learned.

Terry, whose real name is Bobby Joe Horn, was last seen leaving a party in District Heights early one night last summer. His burned-out car has since been located in a rural area near Raleiuh, N.C., but Terry himself has not been located.

For more than a decade, the fasttalking, 41-year-old Terry had been seen by many as the preeminent disc jockey in the world of "personality" black radio in Washington. He worked at WOL for six years, then moved to WHUR where he gained even greater prominence when he talked a confessed killer into surrendering to police. He later was with WEAM and then formed his own entertainment production company.

His disappearance sparked a wideranging investigation, by a federal grand jury and the major crimes division of the U.S. attorney's office here, that has focused on a dispute over the gate receipts from a Maryland soul music concert coproduced by Terry's production company last July 4.

A separate investigation before another federal grand jury in Baltimore resulted yesterday in Terry's indictment for failure to file income tax returns for 1971, 1972, 1973 and 1974.

Investigators aware of both probes said the Baltimore indictment was returned solely to preserve the option of prosecuting Terry on tax charges if he ultimately turns up alive. The statute of limitation expires in April for the alleged tax violation in 1971, investigators said.

Assistant U.S. Attorney Robert P. Trout in Baltimore refused yesterday to commend on whether he has any evidence that Terry is still alive.

"We don't have a body, do we?" Trout said. "If he's found anytime soon, we'll go ahead with the prosecution.But whether he'll show up (for an arraignment scheduled for March 23), your guess is as good as mine."

Investigators connected with the murder investigation, said, however, that it would appear unlikely terry is still alive.

The investigation by the District of Columbia U.S. attorney's office has involved numerous law enforcement agencies. Their agents have found themselves poring through rough financial accountings of the Maryland concert, and quizzing record industry officials in New York. They are seeking to determine whether there is any connection between Terry's disappearance and the still unsolved murders of two District of Columbia men whose bodies were found near the site of the Maryland concert last summer two days before it was staged.

Investigators have made it clear that their probe has not yet led to any solid evidence that Terry was murdered. They have said they still consider themselves to be at a stage in the investigation where they are checking out what they described as theories.

Investigators said the most promising avenue appears to be continued probing of the July 4 concert at the Take-It-Easy Ranch in the Maryland town of Callaway, in St. Mary's Terry's company and another, Charlisma Productions.

Police sources said they have learned that squabbles developed between some principals after the concert over what some backers viewed as a shortage of gate receipts.

Lacking any scientific manner to determine the number of persons who attended the event, investigator's said they do not know with certainly whether the alleged financial shortage existed.

One side in the dispute claimed that as many as 200,000 people attended the event, a turnout that would have grossed at least $1 million, according to one investigator.

The other side maintains, however, that only about 10,000 people showed up - a turnout that would have resulted in huge financial losses for the producers.

Terry's life style immediately before the concert also came under investigative scrutiny, but the only unusual expenditure located for that time period was a new Cadillac that Terry bought for his fiance, one source said.

Foul play was suspected from the time of Terry's disappearance once police determined that he had failed to meet his 13-year-old son at National Airport when the boy arrived from California for a visit. In addition, Terry also had future major production deals in the works and would have been unlikely to abandon them suddenly, investigators said.

One investigator said Terry's disappearance does not appear to be connected with a recent investigation by the Federal Communications Commission into charges that WOL disc jockeys accepted payola from promoters.