One car bomb went off in the parking lot at Columbine High School in the aftermath of the gun chaos.

Another went off in the school and at least 12 others were found in the building.

The discoveries have raised questions not only about the availability of guns to teenagers, but also about how the youngsters could have learned the techniques of creating bombs.

Police made a search of the car park after a device went off

Police who searched the home of one of the dead teenagers - Eric Harris - said they found bomb-making material.

Details of how to make a bomb can be found on the Internet.

After the bombing at the Olympics in 1996 in Atlanta, detectives investigated whether the bomber could have gleaned his ideas from the World Wide Web.

That device was a pipe bomb - as were the Littleton school bombs.

Profiles of terrorist groups on the Web - but more dangerous information is also available

Authorities in the US working to prevent terrorist acts can do nothing about bomb-oriented Web sites.

The Bureau of Alcohol, Tobacco and Firearms admits it receives calls from people concerned about pages explaining potentially-deadly techniques, but cannot take action, because America's First Amendment guarantees freedom of speech.

One school district official said members of the "Trenchcoat Mafia" group were Internet adepts.

Carl Whiteside, director of the Colorado Bureau of Investigation, told BBC News Online that his team were investigating how the teenagers could have learned how to make bombs and got hold of the equipment.

Police powerless

He said devices found so far had been pipe bombs, which were "fairly simple" and did not require a lot of skill to make.

But he believed the boys had a pretty good knowledge of bomb-making to be able to make so many devices, attach a timer to at least one, and smuggle them into the school without them going off prematurely.

"Where they learned about this is still being investigated," Mr Whiteside said. "Anything is possible - but it's a cookbook scenario. They could have got directions or recipes on making bombs from the Internet or by ordering books through the mail.

Pupils running away from the school during the shootings

"There were a lot of devices - we're still trying to determine how many.

"But there's no way this can be policed. it would take resources no law enforcement department has - that's the way of the Internet. My understanding is that one of these guys had his own website.