The discovery also heightens a growing belief that the two young men had help from others in planning for their attack, said the 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 trench coat mafia, to which the two dead suspects belonged, and some members have been given polygraph tests, but so far there have been no arrests, an investigator said.
In trying to piece together the students' motivations and the extent of preparations for the assault, the investigators have seized handwritten notes and a short videotape that the suspects, 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 "jocks" at the school, though the school district here declined today to comment on such a video.
The weapons found at the scene were two sawed-off 12-gauge shotguns, one a double-barrel and the other a pump-action repeater; a 9-millimeter semiautomatic rifle, and a 9-millimeter semiautomatic pistol, said Larry Bettendorf, an agent with the Bureau of Alcohol, Tobacco and Firearms.
Bettendorf said the 9-millimeter weapons were fairly new, and that a preliminary investigation had traced them to a retailer in Colorado. The shotguns appear to be as much as three decades old, so their history could be harder to trace, the agent said.
Bettendorf said he did not know which of the youths owned the various weapons. The bureau is not releasing data on the manufacturers at this time, he said.
Bettendorf said that with rare exceptions sawed-off shotguns are illegal and are used for criminal purposes. To legally possess a shotgun with a barrel shorter than 18 inches, an owner must register the weapon and pay a hefty tax to the A.T.F., he said.
A person must be 18 or older to legally own a rifle or shotgun and at least 21 to own a pistol, the agent said.
As local sheriff'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 to console one another and to leave offerings at makeshift memorials that in places have grown two and a half feet high with flowers. 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."
The superintendent of the Jefferson County School District, Dr. Jane Hammond, said today that the school would remain closed for the rest of this school year, and that school officials were trying to find a place where Columbine High's 1,965 students could finish the year. Students and teachers alike, she 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 assault Tuesday that killed 15 people continued to prompt sorrow and soul-searching around the nation, but it also seems 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 today.
In Wylie, Tex., just northeast of Dallas, a "profane and threatening" statement was found written on a wall of the intermediate school, the police said in a statement, prompting nearly all parents there to take their children out of the school.
In Pleasant Hill, Calif., 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.
And in Newtown, Pa., Council Rock High School will be closed on Friday because of "unsubstantiated rumors circulating in the high school that contained the threat of potential violence," the principal, David Yates, said today.
In Colorado Springs, about 60 miles south of Denver, four teen-agers were charged with trespassing after arriving at school in trench coats and masks in an apparent prank.
Local law-enforcement officials in Littleton said the new bomb at Columbine High today 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" throughout the school had they been detonated. At a news briefing this evening, Sgt. Jim Parr of the Jefferson County Sheriff's office held out the possibility that still more explosive devices could be found as investigators conduct a 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 in the day, Sheriff Stone said that so much explosive material had been found that the authorities believed there was a very good chance that other people were involved in the 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, an official in the sheriff's office said this evening. It was equipped with nails, BB's and broken glass intended as shrapnel and, had it been activated during the busy lunch hour, it could well have killed scores of people in the kitchen and adjoining cafeteria.
In any event, the presence of so much powerful explosive material indicated that the suspects were intent on trying to blow up Columbine High, said Sheriff Stone. "These subjects were not only on a killing rampage, but they were going to destroy the school."
While their full motivation remained a mystery, the suspects' hatred appears in part to have been a grotesque extension of a long-running feud that members of the trench coat mafia had with more popular cliques at the school, known as "jocks" and "preps." New details emerged today of the feud, including descriptions by some students of the taunting to which some of the so-called "jocks" subjected the less popular students, and of a near-brawl last summer that started when one of the trench coat mafia members purportedly flashed a shotgun at the jocks near a local park.
According to these descriptions, the jocks and the trench coat group could act as bullies, the jocks sometimes referring to members of the other group with derogatory homosexual terms and the outcast group sometimes invoking Nazi terminology.
However, most of the students interviewed in the past few days have continued to insist that as nasty as this adolescent feud could be, there was never any widespread fear that it could lead to the kind of horror unleashed here Tuesday.
"I mean, it was just like it must be at every other high school in America," said Breanna Cook, a 17-year-old junior. "You know, kids can be really mean to each other, really cruel. It's always been that way, eventually. But you don't expect it to all turn into a real war."
And no one seemed to know anything about guns and explosive material coming into Columbine High, an assessment shared by law-enforcement officials. "No one saw them carry it into the school and people did see them enter," District Attorney David Thomas of Jefferson County said today, referring to the bomb discovered in the kitchen. "So we have to explore how that device got into the school."
More than 30 people were injured by bullets or shrapnel in the attack on Tuesday and of these, 14 remain hospitalized, with eight listed in critical or serious condition.