A 20-year-old Alexandria man pleaded guilty in circuit court yesterday to the premeditated murder of a Federal Trade Commission economist after the prosecution agreed not to seek the death penalty in his case.

Boggess Steven Davis entered his guilty plea moments before he was to stand trial on the capital murder charge. Davis is subject to a mandatory life prison term as a result of the guilty plea.

While the jury in his case was being selected Monday, Davis pleaded guilty to malicious wounding, two counts of robbery, burglary while armed and three weapons charges, all in connection with the shooting of Steven Marston, 36, and his wife Martha Riche, 44, last January.

If Davis' trial had proceeded yesterday, Riche, who was shot in the upper chest during the incident, was expected to be the prosecution's key witness.

In a preliminary hearing, Riche testified that Davis and two other defendants walked through the open front door of the couple's Parkfairfax town house at 3603 Greenway Pl. at about 9 p.m. on Jan. 2. According to Riche's testimony, Davis held a pistol to her husband's back. Shortly after demanding money, Davis ordered the other two defendants to turn off the lights in the house and then two shots rang out, Riche testified. Marston died at the scene, police said.

Judge Albert Grenadier is scheduled to sentence Davis on July 21. The two other defendants, Jeffrey D. Harris and Davis' brother, Bartel E. Davis, both 17, are scheduled to stand trial as adults in circuit court next month.