The chief Prince George's County prosecutor has shut the door on any prosecutions arising from the alleged operation of a "death squad" within the county police force a dozen years ago.

State's Attorney Arthur A. Marhsall Jr. said his office has "closed its interest" in both the actions of some county detectives who allegedly staged a series of convenience store holdups that resulted in two deaths in 1967 and in an alleged coverup of those events during an internal investigation earlier this year.

"As of this date," Marshall said in a written statement. "I find that the allegations of misconduct lodged against the Prince George's County Police Department . . . are without any credible foundation whatsoever.

Marshall, who was also the county's chief prosecutor in 1967, has has previously issued similar statements regarding the police actions, as reported in The Washington Post last February.

His latest statement was triggered by a letter from Maryland Attorney General Stephen H. Sachs advising that two related matters could not, in Sach's view, result in prosecutions.

These matters included an allegation that police officers directed an informant in 1967 to place stolen tires with persons they wanted to arrest.

"We have no basis to disagree with your judgment that this episode of over a decade ago has no current prosecutional merit," Sachs wrote Marshall Wednesday.

The other allegation concerned possible obstruction of justice during an internal police probe of "the death squad" allegations earlier this year. Sachs said he saw "no substantial evidence to support such charges."

Marshall said in a telephone interview that a police officer involved in the internal probe had allegedly "attempted to influence a former county police officer as to not being perhaps candid."

Following the internal probe, which cleared the police of any wrongdoing, County Executive Lawrence J. Hogan requested a state police investigation. The investigation concluded last month that top-ranking county police officers, using informants to solicit participants, planned and staged a series of holdups in 1967.

The state probers said other officers, who knew nothing about the planning of the crimes, fired the fatal shots.

Marshall said yesterday he believes the charges of police misconduct "never had prosecutorial merit, in 1967 or 1979."