The manager of a High's dairy store staked out by members of a so-called police "death squad" in Prince George's County in 1967 testified today a would-be robber in the store fired his pistol first before officers returned the fire and killed the man in a hail of shotgun pellets.

Bruce Mayben, 47, who said he watched the incident with binoculars from a vacant third-floor apartment across the street, told a federal court jury he saw the robbery suspect point his gun at a policeman posing as a clerk in the store and then saw a flash from the weapon.

"Then I seen flashes come from the ceiling," he said, referring to shotgun blasts from two officers hidden in elevated positions at the rear of the store. The suspect, later identified as William H. Matthews, 18, "fell and was kind of pushed backwards," Mayben said.

"Then I seen a flash from another detective," as Matthews appeared to be crawling toward the door of the store, Mayben said.

Mayben's testimony was viewed as crucial by police in their defense against accusations in a $9 million civil rights lawsuit that they instructed street informants to recruit two unwitting participants for the robbery of the High's store at 9101 Riggs Rd., Adelphi, on June 8, 1967, and then lay "in ambush" for them, killing one and arresting the other.

The three police defendants, including Joseph D. Vasco Jr., now the second-highest ranking official in the county police department, have denied the allegations, contending that informants told them of planned robberies and that detectives then routinely staked out the targeted stores.

Plaintiffs in the complex, three-month-long trial include the families of Matthews and another man, both of whom were shot and killed by police, and two men who were arrested during a series of five holdups and break-ins between June and November 1967.

The plaintiffs contend the robberies and break-ins were deliberately staged by police in violation of the 14th Amendment right to due process.

In testimony Monday, police Capt. Richard Shaner, 43, who had posed as the clerk in the High's store stakeout, said he was unaware of any police scheme to have informants recruit participants for the holdup and plan other details, as alleged by the plaintiffs.

Earlier in the trial, Gregory Gibson, the informant used in the High's holdup, testified Vasco asked him to find participants for the robbery, and retired police officer John R. Cicala testified that the holdup was planned by police "from A to Z."

Mayben, the High's store manager in 1967, who is now a cabinetmaker in Fredericksburg, Va., testified today that because of a rash of holdups at the store, Vasco asked him to allow police to stake out the building after they received information about yet another planned robbery of the place. He said he agreed. Shaner then replaced Mayben as the clerk, and Mayben took up a position across the street to observe the action.