A fresh blanket of snow covers flowers and cards left in memorial to the victims of Tuesday's shooting at Columbine High School RELATED VIDEO: A member of the Trenchcoat Mafia says she doesn't know why her friends did it. From John Ferrugia, of CNN affiliate KMGH. (April 22) Windows Media 28K 80K CNN's Anne McDermott explains the police investigation (April 22) Real 28K 80K Windows Media 28K 80K A profile of the shooting suspects, from CNN's Tony Clark (April 21) Real 28K 80K Windows Media 28K 80K A profile of the shooting suspects, from CNN's Tony Clark (April 21) Real 28K 80K Windows Media 28K 80K To view the latest video streams on the Littleton school shooting, use the arrow buttons below to scroll through the video gallery.        Windows Media Real        28 K 80 K INTERACTIVE: See more scenes from the shooting Audio and Images Interactive Map: Recent school shootings in the U.S. Special: Are schools safe?  ALSO The dead at Columbine High List of injured from Colorado school attack School shooting sparks gun control debate Poll: More parents worried about school safety The aftermath Detailed map of Columbine High School Harris hinted at violence to come  MESSAGE BOARD: Colorado school shooting A faculty member walks into Chatfield High School in Littleton, Colorado, as classes resume Thursday

April 22, 1999
Web posted at: 1:50 p.m. EDT (1750 GMT)

In this story: Did they have help? Suspects' families puzzled, too RELATED STORIES, SITES

LITTLETON, Colorado (CNN) -- Two "large, propane tank bombs" were found Thursday in Columbine High School, a discovery investigators said added evidence to the possibility that other suspects helped the two outcasts who massacred 12 school mates and a teacher before taking their own lives.

Each of the bombs, hidden in duffel bags in the kitchen area, weighed at least 35 pounds (16 kg), authorities said.

Investigators who were inside the suburban Denver school gathering evidence and looking for a motive in Tuesday's gun and bomb assault were immediately evacuated.

"We are seriously looking into the possibility that there were more than two involved because of the amount of bombs, ammunitions and the complexity," Jefferson County Sheriff John Stone said Thursday.

Earlier in the day, school officials announced that Columbine would be closed indefinitely.

For now, the school that became a killing field is a complicated crime scene where the collection of evidence could take days, and cleanup could take weeks.

And some of its 1,965 students, among them victims and witnesses, may also be potential suspects following Tuesday's attack.

With the bodies removed from the school, authorities were expected to release the names of victims -- nine males, four females -- killed by Eric Harris, 18, and Dylan Klebold, 17, before the two high school seniors took their own lives.

All of the dead were students except one male who was a teacher.

More than 20 people were wounded, and most of them remained hospitalized on Thursday, some in critical condition.

Overnight snow that continued into the daylight kept some Jefferson County schools closed on Thursday. But others, except for Columbine, reopened with tightened security.

As they announced on Thursday that the high school would be closed indefinitely, Jefferson County school officials were trying to arrange for other schools in the district to take in Columbine students to enable them to finish the academic year.

Did they have help?

As they searched for a motive, dozens of investigators turned to the community, interviewing acquaintances, classmates and relatives of the gunmen to find out why they did it -- and whether anyone else might be involved.

"That's certainly a possibility," Jefferson County Sheriff's Department spokesman Steve Davis told CNN on Thursday. He added, though, that he did not know "of any concrete evidence at this time to point to a third suspect."

Authorities also have told reporters that they cannot imagine finding a rational explanation for the bloodbath.

Harris and Klebold killed themselves in a second-story library after roaming the halls of their high school with sawed-off shotguns, a semiautomatic rifle, a pistol and homemade bombs stuffed with nails and shotgun shells.

They left behind such a pile of ammunition and explosives that authorities wondered whether they had help.

"That's a lot for two boys to carry," said Stone, who said it's possible there were others involved in making the bombs or bringing them to school.

"It was at least a conspiracy between two people. How much broader than that I do not know," said Jefferson County District Attorney Dave Thomas.

Authorities still aren't sure -- or aren't saying -- where the boys obtained their guns; they made the bombs from materials that could have been bought at most hardware stores.

The probe was complicated by the massive, heavily damaged crime scene -- an entire high school and its grounds -- and the need to catalog every shell casing, every piece of shrapnel, every damaged ceiling tile.

Police also had to photograph the location of every body. The entire investigation is "going to be a long, long tedious job" that will take several days, Davis told CNN on Thursday.

Suspects' families puzzled, too

Members of the "Trenchcoat Mafia" took out an ad in the school yearbook; Harris and Klebold were not in the ad Click here for larger image

Despite wind and cold rain that set in before the snow, hundreds of friends, classmates and relatives kept a vigil late Wednesday night and into Thursday morning near the school, holding candles and flowers and trying to console each other.

The families of the killers issued brief statements that mixed apology with grief and, just as among the victims' relatives, an inability to comprehend the incomprehensible.

"Like the rest of the country, we are struggling to understand why this happened," Klebold's family said.

A statement from the Harris family called the killing spree "a senseless tragedy' and said "Please say prayers for everyone touched by these horrible events."

School officials said they'd had no discipline problems with Klebold or Harris.

Classmates said the two teens were part of an outcast group called the Trenchcoat Mafia and that they wore black and spoke German to each other. They believe their choice of Adolf Hitler's birthday for the attack was no coincidence.

Accounts from students indicated Harris was a leader, a boy who would come to class as if to war, in steel-toed combat boots and a German cross, while Klebold was a follower.

Investigators interviewed the parents of the suspects and searched their homes. They also were interviewing dozens, if not hundreds, of Columbine's students. Some were fellow members of the Trenchcoat Mafia.

Correspondent Greg LaMotte contributed to this report.