The second of a series of former police informants in the Prince George's County police "death squad" trial here testified today that county detectives asked him in 1967 to help them stage a liquor store burglary to lure and catch a long-sought criminal suspect.

Ronald LaVelle, 34, told a federal civil jury that Detective Joseph D. Vasco Jr., now the second highest ranking official in the county police department, and Detective James Fitzpatrick asked him and fellow informant Robert Johnson to get suspect Richard Charles Schoenian "involved in a breaking and entering of Greer's Liquor Store" in Bladensburg.

LaVelle said Vasco and Fitzpatrick set the time and date of the break-in--the early morning hours of Sept. 3, 1967--and indicated they would provide keys for entry and arrange to cut off the store's alarm system.

Schoenian was recruited for the burglary by a third informant, John Crowley, LaVelle testified. On the night of burglary, police waiting at the store arrested Schoenian, LaVelle, Johnson and a 14-year-old juvenile who had tagged along. LaVelle and Johnson were subsequently released, LaVelle said.

In exchange for his help in capturing Schoenian, LaVelle testified, Vasco and Fitzpatrick said they would help him get a light sentence on an unrelated charge of possession of stolen goods. He said they actually did "nothing." He received a three-year probationary term and in 1973 obtained a pardon from Gov. Marvin Mandel.

LaVelle's testimony came in the 10th week of a $9 million civil damage trial against Vasco, Fitzpatrick, and other former and present county officials accused of authorizing and staging five robberies and burglaries in 1967.

In the other incidents, police shot and killed two suspects, wounded a third and arrested several others. Two of the arrested suspects and families of the two slain men filed a $9 million civil suit, claiming the actions of the detectives--called the "death squad" in police circles--violated 14th Amendment right to due process.

Police have denied the allegations, saying the robberies and burglaries were not set up by them. Instead, they say, the informants told them of the planned crimes and police then staked out the targeted stores.

Today, defense attorney James P. Salmon challenged LaVelle's account of the burglary. He noted, for example, that LaVelle testified that Johnson told him that police had selected Greer's for the break-in, but in a deposition given lawyers three months ago, LaVelle said Johnson had not told him who selected the store.