The two students who are believed to have been the gunmen were identified as Eric Harris and Dylan Klebold, students at Columbine High School.
They were found dead of self-inflicted gunshot wounds in the library, said Steve Davis, the spokesman for the Jefferson County Sheriff's Department.
Beginning about 11:30 A.M., the gunmen, wearing ski masks, stalked through the school as they fired semiautomatic weapons at students and teachers and tossed explosives, with one student being hit nine times in the chest by shrapnel, the authorities said. Gunshots continued to ring out at the school for hours. At least 13 bombs were found inside the school. One of them, set on a timer, exploded just before 11 o'clock tonight, officials said. There were no injuries. Three automobiles were rigged with bombs, one of which exploded.
About 3 P.M., hundreds of police officers evacuated the building and searched for the gunmen.
Their bodies and those of several of their victims appeared to have been wired with explosives.
Davis said that as many as 25 people had been killed, both students and faculty members. He said most of the bodies had been found in the school's entrance, the library and the commons cafeteria. No precise death toll was available.
Kaleb Newberry, 16, said: "I was in class and one teacher came in and basically told us to run for our lives, I saw a girl maybe five paces behind me fall.She was shot in the leg. "
Students said the gunmen were part of a group of misfits who called themselves the trench coat mafia, which expressed disdain for racial minorities and athletes. Members of the group found their way out of anonymity at the school by banding together, dressing in dark Gothic-style clothing, including long black coats.
They became easy to notice among the 1,870 students, since every day, regardless of the weather, they wore their coats.
Tuesday the gunmen appeared to aim at minority members and athletes at the school, as well as peers who had poked fun at the group in the past.
School officials had had no reports of trouble from the two group members identified as the gunmen, Davis said. Some victims were forced to wait inside the school for rescue. By early evening, bodies had not been removed because of the crime scene investigation and the possible presence of explosives, a spokesman for the Jefferson County Sheriff's Department said.
Davis said at a news conference Tuesday night that two other students at the school, thought to be friends of the gunmen, were in custody for questioning in connection with the shooting.
"You can't really go in and do this kind of damage without a lot of preparation."Davis said, "without a lot of ammunition and apparently some type of bombs."
Fourteen-year-old Katie Corona, said she was trapped in a classroom with her teacher and about 30 classmates for hours.
"I thought I was going to die," she said. "I really didn't think I was going to make it. We would hear shots, then we heard crying. We had no clue what was going on."
"Everyone around me got shot and I begged him for 10 minutes not to shoot me," one young woman, who was not identified, said Tuesday night in an interview broadcast on the Cable News Network. "And he just put the gun in my face and started laughing and saying it was all because people were mean to him last year."
Another student, who said she heard more gunshots while hiding in the closet with a teacher and some friends, said she kept thinking to herself, "This can't be happening to our school."
"You should be safe at school," added a second young woman, who also was not identified. "This should be a safe place."
The families of those killed were being notified tonight at Leawood Elementary School, where students and parents had gathered.
The mass shooting was the first at an American school during this academic year, but revived memories of similar tragedies that struck six different communities last year and set off national alarms about teen-age violence. Four girls and a teacher were shot to death and 10 people were wounded during a false fire alarm at a middle school in Jonesboro, Ark., last March.
President Clinton immediately dispatched a crisis-response team to aid the school community and the victim's families.
"We don't know yet all the hows or whys of this tragedy; perhaps we may never fully understand it," the President said in a nationally televised news conference just before 8 P.M. "St. Paul reminds us that we all see things all things in this life through a glass darkly, that we only partly understand what is happening." He added, "We do know that we must do more to reach out to our children and teach them to express their anger and to resolve their conflicts with words, not weapons."
A sunny spring day turned into a bloody nightmare for this suburb of 35,000 people southwest of Denver, as ambulances ferried the injured from the high school, past tennis courts, a baseball diamond and a packed student parking lot.
"I hope we can all pull together, because we will need all our strengths," Jane Hammond, Jefferson County superintendent of schools, said tonight. The blood banks in the Denver area have been overwhelmed with calls from donors. Tonight at least three churches held vigils for the dead and injured.
Just before 8 P.M., Lisa Appleton, 16, a sophomore, waited in front of the Leawood school for news of her best friend, Julie Toms, who had been missing since students began leaving the school.
"I can't even feel it," she said. "There's no way to know whether she's dead or alive."
About 3 P.M., SWAT teams police officers used a fire truck and an armored car to get close to the building, dozens of students raced out of the two-story building, some slipping in the mud, others holding up their hands in the air or behind their heads. Police said they feared that the gunmen would try to escape by mingling with the trapped students.
One student, bloodied from an injury, broke out a second-story window, and climbed down into the hands of the police.
As police officers established a wide perimeter around the beige school, students and school workers gathered on nearby tree-lined streets and told of the chaos and horror inside the building.
As fire alarms rang in the halls, students who had seen the gunmen trampled each other to get out of the building, running through one exit where three bodies lay on a staircase.
Trapped inside, others took refuge in classrooms, bathrooms and a choir room, frantically barricading doors with desks and file cabinets.
One cafeteria worker who barricaded herself in a woman's bathroom said, "We could hear them blowing the heck out of the place."
A student, Jonathan Ladd, said, "I heard gunshots going off, bullets ricocheting off lockers."
The trench coat mafia is a small band of about a dozen juniors and seniors at Columbine who are easily recognized yet little feared, people who live in the neighborhood near the school said. Regardless of the weather, they favor long black coats and the Gothic look popularized by the rock singer Marilyn Manson, neighbors said. Some even wear white pancake makeup and dark eyeliner, one student said.
"It was that devilish, half-dead, half-alive look," said Bret, a 16-year-old sophomore who spoke on the condition only his first name be used.
The group often gathered in the cafeteria after school. Chris McCaffrey, manager of Angie's Restaurant a few blocks from Columbine, said that residents had known about the group for about five years and that no one considered it a threat.
"Mostly it was just kids who nobody wanted to have anything to do with," McCaffrey said. "They weren't particularly feared. They were just a bunch of punks who kind of hang around the school."
Students said the group was mostly boys, but that some girls appeared to be closely associated with it. One student described the group as "nerds, geeks and dweebs trying to find someplace to fit in."
Bret suggested the group might have targeted athletes out of resentment for their own lack of popularity and success at school.
The students "weren't really accepted as younger kids and as they got older they were accepted by this group," Bret said. "They got their fair share of being picked on. I could understand that they might have targeted some of the more popular kids."
David Mesch, another student, who was searching for his mother who works for the school, said, "They were wearing masks; they were members of the trench coat mafia."
Several trapped students called television stations when they could not get through on 911 lines.
"I hear a couple of gunshots, people running up and down," a student said in a frightened whisper to KUSA-TV, a Denver television station. Identifying himself only as James, he added, "There are a bunch of kids downstairs, I can hear them crying."
Aware that the gunmen might be watching on a classroom television set, he said, "I am staying upstairs," and then hung up.
In a state with a relatively low crime rate, the siege after the shootings was broadcast live by Denver television stations, without commercial breaks all afternoon. Broadcast and cable networks turned to local affiliates for help with their coverage.
"This is a cultural virus," Gov. Bill Owens of Colorado said before hurrying to the scene. Noting that he felt particularly affected by the tragedy because his 16-year-old daughter goes to a suburban Denver high school, he said, "We have to ask ourselves what kind of children we are raising."
At the White House, the President said the nation should focus on praying for the victims' families and others at the school. He said that Attorney General Janet Reno was closely monitoring the situation, and that he had spoken this afternoon with Governor Owens and Patricia Holloway, the county commissioner, whose comments he shared with the nation.
In midafternoon, police officers briefly detained three young white men who wore camouflaged pants and black jackets, next to the school. After they were released, the men said they knew the gunmen inside the high school. Emblazoned on the back of one man's jacket were the words, "Ban Religion" and a red-painted stop sign printed over a cross.
"Blood was going all over," a shaken girl said, as she was comforted by her father.
Those here were reminded of other school shooting incidents. Bob Sapin, a student, told a television station minutes after watching the shootings, "I just can't believe it is happening at my school."
Other recent shootings involving U.S. schools:
April 16, 1999 -- A high school sophomore fired two shotgun blasts in a school hallway in Notus, Idaho. No one injured.
May 21, 1998 -- Two teen-agers are killed and more than 20 people hurt when a 15-year-old boy allegedly opens fire at high school in Springfield, Ore. His parents are killed at their home. He is awaiting trial. On a police videotape, he is asked why he opened fire. He responds: "I had no other choice."
May 19, 1998 -- Three days before his graduation, an 18-year-old honor student allegedly opens fire in parking lot at high school in Fayetteville, Tenn., killing a classmate who was dating his ex-girlfriend. He is awaiting trial.
April 24, 1998 -- A science teacher is shot to death in front of students at eighth-grade dance at a banquet hall in Edinboro, Pa. A 14-year-old student awaits trial. The motive is unclear.
March 24, 1998 -- Four girls and a teacher are shot to death and 10 people wounded during false fire alarm at middle school in Jonesboro, Ark., when two boys, 11 and 13, open fire from the woods. Both are convicted in juvenile court of murder and can be held up to age 21.
Dec. 1, 1997 -- Three students are killed and five others wounded in a hallway at Heath High School in West Paducah, Ky. One girl is left paralyzed. A 14-year-old student pleaded guilty but mentally ill to murder and is serving life in prison. When asked why he did it, he said he didn't know.
Oct. 1, 1997 -- A 16-year-old boy in Pearl, Miss., is accused of killing his mother, then going to his high school and shooting nine students, two fatally. He has been sentenced to life in prison. The alleged mastermind of the attack awaits trial. Authorities have said the teens were in a cult-like group.