The Maryland State Police have agreed to investigate charges that in 1967 members of the Prince George's County police department operated a unit known as the "Death Squad" which planned a robbery during which a man was killed.

The investigation stems from a series of articles in The Washington Post last month. The stories alleged that on one occasion police ordered an informant to arrange a robbery, picked the site for him, staked out the store and then shot and killed one of the holdup men during the robbery.

Two other stakeouts that year by the same group of detectives resulted in shootings, one of them fatal. In addition, the informant in the first robbery was shot five weeks later by Joseph D. Vasco, the detective who was the squad's second-ranking officer.

Vasco, now a lieutenant colonel and the No. 3 man on the force, arranged one convenience store robbery, according to the informant and another detective. The detective said he was in the car when Vasco the informant that the other holdup men were "going to be blown away."

Vasco has denied the charges and an internal police investigation has cleared police of any wrongdoing. County executive Lawrence J. Hogan then asked the FBI to investigate the allegations, but the FBI refused, saying that the statute of limitations had expired as far as any possible civil rights violations were concerned.

Hogan then asked the state policeto step in.

"Our feeling is the allegations are serio us and the public has a need and a right to know what actually happened," said Willam Clark, spokesman for state police superintendent Col. Thomas Smith.

"We have two things here. If, in fact, some wrongdoing did not take place, the public deserves to have some resolution of that," Clark added. "If there was no wrongdoing, the county police force deserves to have its name cleared."

Clark said that the state police investigators and aides to Attorney General Steven H. Sachs are meeting to decide how to conduct the investigation.

Police chief John W. Rhoads, who ordered the internal investigation, attacked the stories when they were published, saying The Post used "journalistic innuendo," and relied on "unreliable sources," in its reporting.

Sources within the department, however, say that Rhoads now regrets ordering the internal investigation because it raised new questions about the allegations and abouthe investigation.