DISCOVERY OF BOMB ADDS TO MYSTERY.

Byline: Sam Howe Verhovek The New York Times

Bomb squads on Thursday found a large homemade bomb made from two propane tanks and a gasoline canister at Columbine High School, leading the sheriff here to conclude that two students were out to destroy the school Tuesday when they launched a murderous rampage that left 15 people dead.

The discovery also heightened a growing belief that the two young men who stormed the school, firing shotguns and tossing pipe bombs before taking their own lives, had help from others in planning for the attack, said Jefferson County Sheriff John P. Stone.

``They were going to burn the school up,'' said Stone, adding that at least 32 explosive devices and several weapons have now been found at the scene. The authorities have conducted interviews with several students at the school who belonged to the group informally known as the ``Trenchcoat Mafia,'' to which the two dead youths belonged, and some members have been given polygraph tests. So far, there have been no arrests, an investigator said.

In an effort to piece together the students' motivations and the extent of preparations for the assault, the investigators have seized hand-written notes and a short videotape that the two boys, Eric Harris and Dylan Klebold, made last fall for a video production class. Some students have described the video as a macabre preview of the attack in which trench coat-clad students gun down others who take the role of popular athletes at the school, though the school district here declined Thursday to make any comment on such a video.

The investigators also said Thursday that they believed at least some of the weapons used in Tuesday's attack might have been stolen, although they are not sure when. The pair were armed with shotguns whose stocks had been sawed off, a semiautomatic rifle, a pistol and a handgun.

As local Sherriff's Department officers and federal law enforcement officials carried out an investigation they estimated could take weeks, the mourning for the dead continued for another day.

Under a spring storm that alternated between snow and rain, students came to the school grounds Thursday to be together and leave offerings at makeshift memorials that have grown 2-1/2 feet high with flowers in places. But many also questioned whether they would ever want to set foot inside Columbine High again.

``Our friends died in there,'' said Dave Beadey, a 17-year-old junior. ``Even if they cleaned it all up, you'd always know that you were sitting in a place where people got killed. They should just tear it all down and put up a new school someplace else.''

The superintendent of the Jefferson County School District, Jane Hammond, said Thursday that the high school would remain closed for the rest of this school year, and that school officials were trying to find a place where Columbine's nearly 2,000 students could finish the year. Students and teachers alike, Hammond said, had implored them to find a place where they could all do so together.

``We do not want them separated into different settings,'' she said.

The horrific assault Tuesday continued to prompt sorrow and soul-searching around the nation, but it also seemed to have inspired threats of copycat incidents that led the authorities at schools in at least three states - California, Texas and Pennsylvania - to issue warnings or evacuate schools Thursday.

In Pleasant Hill, about 25 miles east of San Francisco, College Park High School was evacuated and a bomb squad called in after a package resembling a bomb was discovered on school grounds.

In Wylie, Texas, just northeast of Dallas, a ``profane and threatening'' statement was found written on a wall of the intermediate school, the police there said, prompting nearly all parents to take their children out of the school.

And in Newtown, Pa., Council Rock High School will be closed today because of ``unsubstantiated rumors circulating in the high school that contained the threat of potential violence,'' the principal, David Yates, said Thursday.

In Colorado Springs, about 60 miles south of Denver, four teen-agers were charged with trespassing after they arrived at school in trench coats and masks in an apparent prank.

The bomb found in the kitchen of Columbine High School on Thursday was made from two 20-pound propane tanks similar to those used with household grills, along with a gasoline-filled canister, which could have caused ``an extensive amount of damage'' had they been detonated, Sgt. Jim Parr of the Jefferson County Sheriff's Department said at a news briefing Thursday night. Parr held out the possibility that still more explosive devices could be found as investigators conduct an ``inch-by-inch'' search of the school.

``I'm not going to say that all the bombs are out of the school,'' he said. ``It's a very large building.''

Earlier on Thursday, Stone said so much explosive material had been found that the authorities believed there was a good chance that other people were involved in the deadly attack.

``It's drawing suspicion out here that they would have time to put as much ordnance in that school as they did without some help,'' the sheriff said.

The bomb was found in a large duffel bag hidden in the kitchen and was attached to what might have been a remote-controlled detonation device, a Sheriff's Department official said Thursday night. It was equipped with nails, ball bearings and broken glass intended as shrapnel and, had it been activated during the busy lunch hour, could have killed scores of people in the kitchen and adjoining cafeteria.

The presence of so much powerful explosive material indicated the suspects were intent on trying to blow up Columbine High, Stone said. ``These subjects were not only on a killing rampage, but they were going to destroy the school,'' the sheriff said.