One of three men charged with murdering a prisoner during a daring break-in at the Lorton prison complex has pleaded guilty to a lesser charge and is expected to testify against the other men charged in the bizarre incident.

The surprise plea by John Elbert Landon, 30, of 4407 Lions St., Marlow Heights, came a day after he and the other men were indicted by a federal grand jury in Alexandria on murder and conspiracy charges.

Court sources said yesterday that prosecutors had agreed to drop the indictment against Landon in return for his plea to a charge of assault with intent to commit murder and his cooperation with their investigation.

Prosecutors declined all questions about the plea agreement yesterday, but others familiar with the case said Landon was to testify at the upcoming trials of Edward Ford Sharrieff, 32, of 16 S. Van Buren St. Alexandria, and Germain P. Stoddard, 24, 409 Upshur St. NW.

All three men are accused of breaking into the District of Columbia's minimum security facility in Fairfax County May 23 and gunning down convicted drug dealer Douglas M. Boney at point-blank range.

The incident, which is still under investigation, is believed by police to be related to drug trafficking in the Washington metropolitan area and the prison itself. The three men were arrested hours after the shooting by police in Fairfax County. The prison where the shooting occurred is 20 miles south of the District and is a mile from the main prison compound.

"We're looking for two snitches," the intruders allegedly yelled at 3:15 a.m. as they forced several dozen prison inmates to line up against the wall during the incident. Officials later said the men were seeking Boney and John D. Irby, a convicted murderer, who once worked in a prison work-release program. Irby, who escaped injury, once worked in an auto repair shop owned by Boney's brother, Edward, in Washington.

Boney, 35, was first shot as he attempted to bolt from a prison building, and then shot a second time at point blank range by Sharrieff, assistant U.S. Attorney Karen P. Tandy said at a court hearing.

Boney was serving a 2-to-6 year sentence for heroin distribution at the time of his death. Sharrieff was on probation for a drug conviction at the time of the shooting, according to Tandy.

Irby is regarded by law enforcement officials as a main conduit for illegal drug traffic at Lorton.

Two aspects of the case continue to puzzle investigators. One is the possible connection between the Lorton shooting and the deaths of six other people in the District in drug-related incidents.

The other is the fact that FBI agents traced the ownership of the alleged getaway car to the home of a D. C. Department of Corrections counselor.

The counselor, Robert (Pappy) Gault, acknowledged knowing Sharrieff and Boney, but said the 1980 Cadillac was registered to his Lorton address without his permission.

Landon faces a maximum penalty of 20 years in prison for his guilty plea which was entered Tuesday. He will be sentenced on July 25.

Sharrieff and Stoddard are scheduled to be arraigned on their indictments Monday. No trial date has been set for them. Each faces a maximum of life in prison if convicted on the murder charges.