Among the items found in Evan Ebel's black Cadillac were instructions on how to make a bomb, black powder, a projectile and four cartridge cases, ABC News reported.
Authorities in Texas, where Ebel died in a shootout with police, also recovered a pair of tan pants "with apparent blood" from a cooler in the trunk, an evidence log indicated.
Ballistics testing showed that the gun found on Ebel was the same one used to kill Clements, who was shot March 19 when he answered his front door in Monument, Colo.
Texas officials said Tuesday Ebel had a backpack that contained Colorado Department of Corrections documents and three pages of handwritten directions that included Clements' home address.
"This young man was on a mission," said Wise County Sheriff David Walker.