Maryland State Police, investigating a rock-throwing incident early Sunday that injured seven people on the Capital Beltway, said yesterday investigators were "following very promising leads" after receiving calls from people with information about the incident.

"We've had some civilians call in to say they think they knew who did it," said 1st Sgt. Doug MacLean.

"The information appears to be very promising in that they've been busy with it virtually from the word go this morning."

State Police were looking for three youths from 14 to 18 years old who witnesses said were standing in the traffic lanes of the Capital Beltway near Livingston Road in Oxon Hill about 2:30 a.m., throwing rocks at cars.

State Police investigators yesterday went to four Prince George's County high schools to ask for cooperation from the principals and to interview students, MacLean said.

During the incident, at least 25 vehicles were struck with gray landscaping stones weighing five to 15 pounds, police said.

The most seriously injured, Destiny Morris, 15, of Hagerstown, Md., remained in critical condition yesterday in intensive care at Southern Maryland Hospital Center.

Morris, who was on the way home after a holiday weekend at Ocean City, Md., was struck in the head by a rock that crashed through the window of the car in which she was riding, police said.

Kelly Moody, 21, of Manassas, the only other motorist hospitalized, was released from Prince William General Hospital on Monday. Moody's mother, Karen Moody, said that her daughter had asked repeatedly about the condition of Destiny Morris.

"I know that Kelly feels very badly about the other girl," said Karen Moody. "We're praying for Destiny."

The incident prompted angry reactions yesterday from Gov. William Donald Schaefer and Prince George's County Executive Parris Glendening, who pledged state and local efforts to obtain an arrest.

"I hope these individuals are arrested and prosecuted to the fullest extent of the law," said Glendening.

"We don't need these kinds of citizens in Prince George's County."

Investigators yesterday concentrated their search on two apartment complexes near the Beltway and Livingston Road in southern Prince George's after receiving tips that the youths lived there, MacLean said.

Police also were reviewing five similar rock-throwing incidents that had occurred during the last six months on the assumption that the incident Sunday was not a first-time offense for the youths involved, MacLean said.

In one of those cases, a woman who contacted The Washington Post said that her boyfriend, who was taking pictures of an abandoned house at the time, photographed youths who threw a gray landscaping stone at her car on May 24 near Livingston Road. The woman said she reported the incident to the Forest Heights Police Department.

"There was a gang of about 15 to 20 kids and then the next thing we knew, this rock came flying down and hit my car and bounced off and hit a dog in this woman's yard," the woman said.

Several other people who called The Post recounted similar incidents and said they felt at the time that their complaints were not taken seriously by State Police.

Susan Richards said that she waited for an hour to make a report after youths threw large pieces of wood at her car on Oct. 27 as she was driving on the Beltway near Livingston Road.

The debris broke the windshield in Richards's car and caused $800 in damage, she said.

"I could have easily been killed or in a bad accident and no one ever called me to follow up at all," said Richards.

"They were very nonchalant about the whole thing and it was all I could get them to do to take a report over the telephone."

Larry Silas, 36, a District resident, said that State Police officers at the Forestville barracks refused to take a report from him after the car in which he was riding was struck by a brick thrown by youths on Route 4 inside the Beltway on March 18.

Silas said that troopers told him to call Prince George's County police.

"The State Police were completely unsympathetic, so when I heard about this incident this weekend, it upset me tremendously," said Silas.

Elliot Dalberg, 40, an advertising executive from Alexandria, said he was driving north of Route 4 on the Beltway late one night last summer when a bullet-sized rock struck the curved windshield of his Saab from the side. When Dalberg reported the incident to State Police in Forestville, he said troopers "accepted it very calmly" and told him that rock-throwing was "very commonplace" on that stretch of the Beltway.

MacLean said that State Police take reports of rock-throwing "very seriously," but also noted that frequently roadside debris is accidentally thrown into cars by other vehicles.

"If we look at your car and determine there is damage and that it came from someone throwing a rock or whatever, then we would make a report," said MacLean.

Capt. William Brooks, who oversees State Police barracks in Forestville, College Park and Rockville, estimated yesterday that there had been a half dozen serious rock-throwing incidents on the Beltway since September 1988, but said that the State Police do not keep statistics.

Police have characterized Sunday's incident as the worst in recent memory on the Beltway.

Most of the incidents reported by police and by callers occurred on the Beltway between Indian Head Highway and Livingston Road and between Routes 214 and 50.

On Sunday, as in several previous incidents, the rock-throwers hurled heavy landscaping stones used to prevent erosion along the Beltway overpass over Livingston Road, further leading police to suspect that the youths are local residents, MacLean said.

"Our best information is pointing to kids that live in the area," said MacLean. "You don't know the rocks are there just by happening by the scene.

"It has to be someone familiar with the area."