Difficult and Rude

Michelle Hartsough, a friend and co-worker, told investigators that she had learned a lot from her father, a pyrotechnics expert.

Hartsough showed Harris and Klebold how to remove “black,” or explosive powder, from fireworks and use it for other pyrotechnics effects, she said. Littleton fire officials have said some of the killers’ many bombs were made from fireworks explosives.

For a year, Hartsough listened to the boy’s tales of killing the jocks but said she never took them seriously.

“She said they both talked about a senior prank that was going to take place April 20, 1999,” the report said.

When the kids were bored, they would buy dry ice at the nearby Baskin-Robbins ice cream store and make little bombs they detonated behind the Blackjack. Harris used fireworks to build a “trip-wire bomb” to keep kids from going through a hole in the fence.

Klebold was once briefly fired for bringing a pipe bomb to work, the report said. He was hired back when the owner needed workers. 
Source