Crunch is very right on how traceable explosives are. That being said tracing it to an individual is very hard to do. If one guy blows up multiple people over several incidents, then it gets a lot easier. Without revealing the actual technique to tracking explosives, one guy usually uses the same materials, wires the bombs the same, etc.
Also, to look at a real life hitman, Robert Pronge used remote detonated grenades under cars for years. He never got caught. How do I know about him? Another hitman (Richard Kuklinski) killed him. Remote explosives don't have to be indiscriminate, you just have to be careful.
@Joker: A surprisingly few companies make explosives and they all leave "prints". Very easy to trace an explosive back to a company, even after detonation.
CEXC teams (Combined Explosives eXploitation Cells) are REALLY good at tracking explosives.