Prince George's County Assistant Police Chief Joseph D. Vasco Jr. introduced a memo in federal court today that appeared to contradict accusations that he directed an informer in 1967 to recruit participants for a robbery in which one of the participants was shot and killed by waiting police.

Testifying for the first time in the five-month-long "death squad" civil rights lawsuit here against county police, Vasco, 47, said informer Gregory Gibson approached him in early June 1967 with information that two Takoma Park brothers, Marvin and Mervin Rozier, wanted to rob a store, but had asked Gibson to select the site and "case it" first.

Vasco, then a rank-and-file detective and now second highest ranking official of the 900-member county police force, said he immediately typed a memo, recounting Gibson's words in narrative, first-person form. He said he discovered a carbon copy of the memo among personal papers at his home 12 years later in 1979 shortly after plaintiffs in the current civil rights trial sued him and two other officers for $9 million.

The 143-word memo, considered crucial to the case by police attorneys, was introduced into evidence today and read to the six-member civil jury hearing the case.

"I met with Marvin and Mervin Rozier at their house. . . " the memo says in part. "They suggested that the three of us go out and hold up a store tonight. They told me to pick a place and case it and then let them know where."

The memo contrasts sharply with testimony earlier in the trial by both Gibson and a retired police officer, John R. Cicala. Gibson said Vasco instructed him to recruit associates, and Cicala said he overheard Vasco tell Gibson to get a "couple of patsies" for the holdup.

Gibson and Cicala also testified that Vasco selected a High's Dairy store in Adelphi as the targeted site and obtained a car for Gibson and his associates ostensibly as a getaway car. The robbery was "planned from A to Z" by police, Cicala said.

The memo acknowledges that Vasco helped Gibson select the site, and Vasco acknowledged in testimony that police arranged for the getaway car. Vasco and other officers have contended that site selection can be justified by police to minimize possible harm to passersby and that providing a car or other "instrumentality" to persons with a criminal predisposition is constitutionally permissible.

On the night of the High's robbery, June 8, l967, waiting police shot and killed William Hunter Matthews, 18, and arrested Marvin Rozier. Gibson was allowed to escape on foot. Matthews, who had been recruited in place of Mervin Rozier at the last moment, was shot when he ignored orders to halt and fired his .32-caliber pistol at police, according to Vasco and other officers.

Marvin Rozier and Matthews' family, plus the family of a second man shot and killed in another so-called "death squad" incident in 1967, sued Vasco and other officers in 1979, claiming police violated their civil and constitutional rights by staging the robberies. The suit followed published reports of the "death squad" allegations in The Washington Post.