The stepfather of a teenager charged with throwing stones at cars on the Capital Beltway testified yesterday that his stepson returned home on May 27 about 15 minutes before witnesses say the violent rock-throwing spree began.

Raymond Thomas Johnson, of Forest Heights, said his stepson, John Lavon Burgess, 18, came home about 1:45 a.m. after an outing with friends. Johnson said he was certain of the time because his stepson had a 1 a.m. curfew and he "looked at the clock to see what time it was."

The prosecution had earlier introduced a signed statement in which Burgess told police that he and two friends threw the rocks "just for fun." A prosecutor said yesterday he intends to argue that the rock-throwing incident near Indian Head Highway, which disabled 24 cars and left a 16-year-old girl in a coma, was premeditated and that Burgess and two other defendants may have thrown rocks from the Beltway on other occasions.

Johnson's testimony came as Prince George's County Circuit Judge William Missouri dismissed 22 charges in a 90-count indictment against Burgess, the first of three young defendants to stand trial on assault and property destruction charges in the incident. The two others are Maurice Edward Ford and Donnell Petite, both 18.

The most serious charges are assault with intent to murder Destiny Morris, 16, of Hagerstown, Md., and six others who were passengers in cars.

Missouri, who said he will issue a verdict today in Burgess's case, said the state failed to provide sufficient evidence on 17 of the charges. Five more were dropped because a victim failed to testify.

More than 20 victims of the rock-throwing incident testified during the two-day non-jury trial that their cars were attacked between 2 and 3 a.m. by three people who ran from the sides of the Beltway and hurled small boulders at their windshields and windows. None of the witnesses could identify Burgess.

In some cases, the motorists said they were lured to the highway's left lanes by the three, who waved their arms or lay down in the road to attract attention.

The prosecution closed its case yesterday with testimony from Burgess's friend Calvin Morrison, of Greenbelt, who said he was with the three the night before the incident.

Around 1 a.m. on May 26, Morrison -- whose nickname is "Madball" -- said the three led him under a Beltway overpass, a frequent shortcut on their way home. He said Ford picked up a stone, tossed it in the air and said to the others, "Let's introduce Madball to the bridge."

John Smathers, assistant Prince George's state's attorney, told reporters that the comment shows "premeditation. I'm going to argue that it means they've done this before and that they thought about it."