Plaintiffs' attorneys in the Prince George's County police "death squad" trial sought today to show that an admitted housebreaker and one-time police informer was pressured by police last December not to testify against officers in the trial currently being held here.

The informer, using the pseudonym Sandy Stillwell, said an "associate" of his facing criminal charges in Montgomery County relayed a message to him that Prince George's County police wanted to "involve" Stillwell in criminal activity so that he could be arrested.

At the time, Stillwell was on call to testify against three current and retired county police officials, including Assistant Chief Joseph D. Vasco Jr., who are accused in a $9 million lawsuit of illegally directing informers to recruit participants for a series of convenience store robberies and break-ins in 1967. The suit alleges that police waiting at the stores shot and killed two participants, wounded one and arrested seven others.

Police have denied the accusations, saying the informers came to them with tips about planned holdups, and detectives then routinely staked out the targeted stores. Pistol-wielding suspects were killed or wounded, they said, only after they ignored orders to halt and threatened the lives of police.

Stillwell, 34, was one of several informers involved in the incidents. He was never called as a witness against police in December but is now on the stand on behalf of police in the four-month-long "death squad" trial here.

He has denied that police instructed him to recruit participants for crimes. But on cross-examination by attorneys for the plaintiffs against the police, he acknowledged telling Maryland State Police investigating the "death squad" allegations in 1979 that Vasco and others did ask him to recruit acquaintances to burglarize a Bladensburg liquor store on Sept. 3, 1967.

However, Stillwell said today he lied to state police because, since his covert informer role had been revealed, he wanted to protect himself from possible reprisals by suggesting county police had coerced him into the role.

Next, Stillwell said under questioning by the plaintiffs' attorney, Barnet D. Skolnik, that his Montgomery County "associate" warned him that Prince George's County police wanted to "involve" him in a crime last December before he was to testify for the plaintiffs. Stillwell denied telling Skolnik that, if called to the stand, he would lie and "possibly act crazy" on the witness stand.