It is the random quality of the attack that angers Ellis Morris, makes him feel helpless and sick. One minute his teenage daughter, Destiny, was happily returning home with family friends after a visit to Ocean City, Md. Then, she was fighting for her life, her skull fractured in a bizarre rock-throwing attack against motorists on the Capital Beltway early Sunday.

"It makes you wonder just how cruel people can be," said Morris, of Hagerstown, Md., as he kept a vigil at his daughter's bedside at Southern Maryland Hospital Center in Clinton. "It makes you wonder just how you can protect your child."

Destiny Morris, 15, who was hit by a heavy rock that flew through the windshield as she slept, was unconscious and listed in critical condition yesterday. She is the most seriously injured of the seven people who were hurt in the incident near Oxon Hill. At least 25 cars were attacked with large landscaping rocks, weighing five to 15 pounds, in the Livingston Road area.

Maryland State Police are looking for three young males, between 14 and 18, who apparently ran into traffic to heave the rocks and, according to some witnesses, even tricked motorists into slowing down by lying in the road, then jumped up and hurled the missiles.

Police said yesterday that they have received dozens of tips about the case, which carries charges of attempted murder, destruction of property and assault and battery, but have no suspects. In the meantime, they have beefed up patrols on the Oxon Hill section of the Beltway, Sgt. Robert Jones said.

Rock throwing is not uncommon in the Washington area, especially on interstates, but an incident of this magnitude, involving this many vehicles, is rare, police said. Uusually, rock throwing begins as a prank by bored teenagers, police said, but sometimes there are deadly consequences.

"We're surprised -- and glad -- that more people weren't hurt," Jones said.

In June 1988, a Takoma Park man who was driving home on Sligo Creek Parkway, bearing a takeout dinner for his family, was killed when a 36-pound boulder was dropped onto his car from a bridge. A 16-year-old Silver Spring youth was charged in the case.

In December 1977, Maryland State Police closed a portion of the Baltimore-Washington Parkway for several hours after five early morning motorists were pounded with rocks.

A year earlier, in the same area, a passenger on a Greyhound bus was seriously injured when a stone shattered a bus window. During that one-year period, about 15 rock-throwing incidents were reported in that area, police said.

In a 1977 interview with The Washington Post, a group of Prince George's County teenagers who lived near the parkway tried to explain why they liked to throw rocks at passing cars. "It's just something to do," said a 14-year-old. "We usually try to see how many cars we can hit and how many windows we can break . . . "

When asked about the danger to drivers and passengers, he said, "Oh, the people . . . we don't really think about them."

Several people whose cars were hit early Sunday said they still feel dazed, unsure about what exactly happened. Gloria Scott, whose friend, Kelly Moody, hurt her chin and chest, said she initially thought she had blown a tire when she heard a loud "pow" as they made their way home from a nightclub. Rodney Marbury, 27, of Landover, told his mother, Delores, that he slowed down because of a commotion ahead and suddenly saw a young man hurl a rock into his window. Marbury's arm was broken.

David Powell, an Air Force cadet, said he saw no threatening figures on that stretch of the Beltway as he returned home to Dale City after a day in Annapolis.

But someone hurled a 10-inch rock through his window, shattering the glass and bruising his shoulder.

"A lot of folks around me are outraged," Powell said yesterday. "But I still haven't gotten over the fact that I wasn't hurt worse."