Attorneys accusing members of a so-called police "death squad" in Prince George's County of ambushing unwitting suspects in a series of staged holdups in 1967 sought today to discredit a key eyewitness for police, contending his view of one of the holdups was substantially blocked.

Bruce Mayben, 47, manager of a High's Dairy store in Adelphi that had been staked out by police, testified in federal court earlier this week that from his view in an apartment across the street, he saw a would-be robber fire his pistol before waiting police returned the fire and killed him.

Today, under cross examination, Mayben acknowledged that windows in the store were partly blocked by stacks of merchandise and he was three times as far away from the store as he originally testified--225 feet, rather than 75 to 80.

Nevertheless, Mayben said, he was able, using binoculars, to see one of two suspects, later identified as William H. Matthews, 18, point his pistol at a policeman posing as a clerk in the store and then turn and fire toward the rear of the store. Mayben said that shot was followed by shotgun blasts from police hidden in the rear.

A second witness in the apartment, Robert Lloyd Brooks, 34, a former High's employe, testified today that he heard a small-caliber shot first and then the blast of shotguns.

"I remember hearing small fire before the big fire," he said.

The testimony of the two witnesses is crucial to both sides in the $9 million civil rights trial currently in its third month here against three present and retired police officials, known by some in police circles as members of the "death squad."

The three, including the current No. 2 man in the police department, Col. Joseph D. Vasco Jr., are accused of instructing informants to recruit participants for a series of five robberies and burglaries and then assigning officers to lie "in ambush" for them at the targeted stores.

In the five incidents between June and November 1967, two suspects were shot and killed, one was wounded and seven arrested by waiting officers.

Two of those arrested, plus the families of the two slain men, sued police for $9 million, alleging deprivation of civil rights.

Police deny they directed informants to recruit participants, maintaining the informants came to them with tips about planned robberies and burglaries.

In the High's store incident, which occurred on the night of June 8, 1967, official police reports say officers shot and killed Matthews only after he ignored an order to halt and fired toward officers hidden in the rear of the store