A key witness in the so-called police "death squad" trial here acknowledged today he signed an affidavit two years ago that defense lawyers say contradicts his claim that Prince George's County police directed him to recruit participants for a robbery in which one of the participants was shot to death.

Gregory Gibson, 33, a police informant at the time of the robbery in 1967, acknowledged in federal court the affidavit says two of his friends "were ready" to commit the robbery themselves. But he added that he does not recall using that language when a defense attorney interviewed him for the affidavit in October 1980.

The issue of whether Gibson recruited the participants at the behest of police or the participants acted on their own initiative is a central factor in the current trial.

Survivors of two men shot and killed by police in a series of robberies in 1967 plus two other men arrested in the incidents filed a $9 million wrongful-death lawsuit against the police and other county officials, contending the robberies were staged by "death squad" members in a crackdown on convenience store holdups. The suit claims that police directed Gibson and other informants to recruit participants for the holdups.

Police deny the allegations, contending Gibson and the other informants simply reported planned robberies to them, and police then staked out the targeted stores.

The three alleged members of the "death squad" are Lt. Col. Joseph D. Vasco Jr., now the second highest ranking official of the county police department, Capt. James Fitzpatrick, commander of the major crimes division, and retired Lt. Blair Montgomery.

Prince George's County State's Attorney Arthur A. Marshall Jr. and former Assistant State's Attorney Benjamin R. Wolman are also accused in the suit of authorizing or acquiescing in the "death squad" stakeouts.

Last week, Gibson testified that, under instructions from Vasco, he contacted two friends, twin brothers Marvin and Mervin Rozier, about the rash of holdups in the spring of 1967.

When they said they were not responsible, Gibson said Vasco asked him, "Do you think you can get them to do one?"

Gibson said he returned to the Roziers and "convinced them" to commit a robbery. He said Vasco then selected the date, time and location--a High's Dairy store in Adelphi--and provided Gibson a getaway car.

As it turned out, Mervin Rozier did not participate in the holdup. Marvin Rozier and a second man, William Hunter Matthews, ended up accompanying Gibson to the store. In the incident, on June 8, 1967, Matthews was shot and killed by police. Rozier was arrested unharmed.

Matthews had been armed with a pistol and fired at police when ordered to halt, according to defense attorneys.

Today, Gibson acknowledged that portions of an affidavit he signed for Assistant Attorney General Stephen B. Caplis on behalf of Marshall and Wolman in 1980 differed from his account last week.

"Didn't you tell Caplis it was the Roziers' idea to do the robbery?" asked defense attorney James P. Salmon

At first, Gibson said, "I don't recollect." Later, he said, "we had a conversation concerning that point. . . and I told Caplis it didn't take much persuasion to get them to do one."

The affidavit has not been introduced as evidence in the trial, and the jury has not seen its text.