Prince George's County Executive Lawrence J. Hogan yesterday asked the Justice Department and the FBI to investigate allegations that a small group of county police known as the "Death Squad" set up an armed robbery in 1967 during which a man was killed.

Hogan said that, although the county's own recent investigation contradicts these allegations, a separate investigation by neutral outsiders would "clear the air once and for all" on questions raised by recent articles in The Washington Post.

In a prepared statement released yesterday, Hogan said: "If the information presented by The Washington Post is true, there could have been violations of several civil rights statutes.

"I am therefore requesting the Federal Bureau of Investigation to investigate these matters, resolve the discrepancies and clear the air once and for all."

Both a former detective on the so-called "Death Squad" and a police informant who worked with them told The Post virtually identical stories about a June 1967 armed robbery -- in which an 18-year-old was killed -- which they say was set up by police.

That robbery and two others in 1967 ended in police shootings, including one other fatality.The police actions that resulted in the deaths, according to the former detective and others involved, were taken to stop a rash of convenience store holdups in the county.

"The accounts published in The Washington Post," County Executive Hogan said yesterday, "are, of course, disturbing. However, an exhaustive investigation conducted by an associate county attorney revealed evidence which contradicts the information published by The Washington Post.

"It is not surprising that there are discrepancies when recollections of witnesses are 11 or 12 years old," Hogan said.

Yesterday morning, Jack McBride, a Hogan aide, contacted both the FBI Baltimore field office and Justice's Civil Rights division. He said both told him that too much time may have elapsed to prosecute possible crimes committed in 1967, under most federal laws.

The federal civil rights laws to which Hogan alluded cover a multitude of crimes, including volations of a person's constitutional rights to due process and equal protection under the laws.

Hogan said if Justice declines to investigate he will likely ask "some other disinterested third party, possible the state police," to do so. "The story raised some legitimate questions that should be resolved," Hogan added, in a telephone interview.

In a statement issued yesterday morning, Police Chief John W. Rhoads said The Post had "seen fit to attack the Prince George's County police department with what appears to be uncorroborated statements by unreliable sources."

Rhoads said The Post had "suggested that a 'Death Squad' existed... through the clever use of journalistic innuendo."

Rhoads and associate county attorney John B. Wynes had earlier provided The Post with the results of the investigation: an 11-page report, transcripts of 10 interviews and other documents on which they based their conclusions.

Yesterday, Rhoads refused to release the final 30-page report or transcripts of subsequent interviews.

Rhoads met yesterday with the leaders of the police union, the Fraternal Order of Police, to seek their support. The FOP board voted to "stay in the middle until all the facts are in," President Laney Hester said later.

The armed robbery of an Adelphi High's occurred June 8, 1967, according to former Det. John R. Cicala and informant Gregory Gibson, under a plan set in motion by Joseph D. Vasco who has been second in command of the entire department for the last three years.

Cicala and Gibson said Vasco picked the location and the time and arranged for the getaway car. Vasco vehemently has denied any role in planning the holdup.

Killed during the police stake-out of the armed robbery was William H. Mathews Jr., 18, a Takoma Park man police said was a dangerous criminal, according to police informant Gibson and another policeman who was a "Death Squad" member.

Gibson told The Post recently that he encouraged Mathews and Marvin P. Rozier to join in the holdup with him after receiving instructions from Vasco to "get me a couple of patsies."

There is no public record of any prior criminal involvement by Mathews in Prince George's County. Yesterday, his mother Mary Kosh, described her son as a law-abiding youth whose only brush with the law had been a disorderly conduct charge in Montgomery County.

"After he got killed, I heard he got set up by the Gibson boy, but I had no idea the police were involved at all," she said, at the Takoma Park home of her ex-husband, Mathews' father.

"To me, they shot my boy down like a dog," said William H. Mathews Sr. "Police officers who are supposed to uphold the law acted as judge and jury."

With them was Charles L. Richards, Mrs. Kosh's attorney. Mathews and Kosh have discussed possible legal action as a result of the recently revealed allegations, but have made no final decision.

"Right now," Mathews said, "I don't have malice against anybody. I want to find out the truth. I just want to know why he paid the price he did."

Their son, they said, had completed 11th grade at Montgomery-Blair High School where he played football. After his senior year season, Mrs. Kosh said, "all of a sudden he decided he didn't want to go no more." He would have graduated the following June, the month he died at the High's store.

He had mentioned a desire to play professional football, his mother said. Mathews, a bricklayer, said his son "probably would have" taken up his father's trade.

As for Gibson, the informant, Mathews' mothe said, "he did wrong, but I also feel they set him up, too."

Rhoads singled out Gibson, although not by name, as an unworthy source for The Post articles. During the late 1960s, he said, the county police were "dealing with some very dangerous people who possessed a limited capacity for fear. People of this type who would lose no sleep over the death of a stranger or, as evidence by these incidents, a friend who will do and say whatever suits their purposes at the time."

Rhoads further asserted that "the state's attorney was totally apprised of each incident both before and after the fact." Arthur A. Marshall Jr. Prince George's prosecutor since 1963, said yesterday he had "no recollection" of such notice, "before or after."