A federal judge today freed the chief prosecutor in Prince George's County and one of his former assistants from the so-called police "death squad" civil rights lawsuit. In the first ruling of its kind during the three-month-long trial, U.S. District Court Judge Herbert F. Murray ruled that the men's accusers had failed to prove the two acted "in concert" with police in a series of allegedly staged robberies in 1967 in which two suspects were shot and killed.

Murray directed verdicts in favor of State's Attorney Arthur A. Marshall Jr. and Benjamin R. Wolman, a former part-time prosecutor, as the plaintiffs rested their case.

But the judge turned down motions for directed verdicts on behalf of three present or retired county police officials who also are plaintiffs in the $9 million lawsuit, including Assistant Chief Joseph D. Vasco Jr. Murray held that the evidence against the three, if believed by the six-member civil jury that has been hearing the case since last November, could be used in considering possible damages against them.

The judge also rejected motions for a directed verdict on behalf of the Prince George's County government, thus keeping alive claims by the plaintiffs that the county failed in its responsibility to supervise and discipline police in order to prevent the so-called "death squad" incidents.

Outside the federal courthouse here, Marshall and Wolman told reporters they were happy with the judge's action. "I'm pleased it's over," said Marshall, but he added he is now considering a possible countersuit against the plaintiffs for what he called "malicious prosecution."

The trial, involving incidents going back 15 years, was wrong, Marshall said, and "should never be allowed to happen again."

The "death squad incidents did not come to public light until 1979, when The Washington Post published a series of articles containing allegations about the "death squad." The lawsuit was filed subsequent to the articles' publication.

Wolman, accompanied by his wife, Mary, said, "I am pleased that Judge Murray put the pieces together and made the decision he did."

In his ruling today, Judge Murray said outside the presence of the jury that there was "no legally sufficient evidence" in the trial that either Marshall or Wolman was part of an alleged scheme whereby police allegedly instructed informants to recruit participants for a series of five robberies and burglaries of businesses in the Hyattsville area in 1967.

There was "not an iota" of evidence that the two prosecutors knew personally about the alleged improprieties, Murray said, and since it was not their job to supervise the police, they also could not be sued on the theory that they "should have known" of the alleged improprieties.

Attorneys for the plaintiffs had argued that Marshall regularly advised police about field procedures and that Wolman observed one of the shooting incidents with a pair of binoculars. Murray ruled, however, that they nevertheless lacked direct knowledge of any improper actions.

The plaintiffs in the case--two men arrested in the incidents and the families of two others killed by police--contend that Vasco, Capt. James Fitzpatrick and retired Maj. Blair Montgomery were members of a group of detectives in 1967 known in police circles as the "death squad."

The plaintiffs contended that in a drive to curb a rash of convenience store robberies, the three detectives directed street informants to recruit unwitting associates to participate in a series of five holdups and break-ins between June and November 1967. During those incidents, waiting police shot and killed two suspects, wounded a third and arrested seven others, according to the plaintiffs.

These actions violated the suspects' civil rights, including the right to due process under the 14th Amendment, the plaintiffs contended.

Police have denied the allegations, countering that they received information about planned holdups and break-ins from the informants and then routinely staked out the targeted stores.

Vasco, Fitzpatrick and Montgomery are expected to testify in detail about their roles in the incidents and their relationship with the informants.