Some years back I worked on a couple of programs that were looking into ways to harden high value targets. Bombs were an obvious avenue of attack, so we looked into the records from various government agencies (FAA, FBI, ATF, etc.).

Bombings that were grudge motivated (my term), such as "judge decided against me," "bast@rds fired me," "dirt bag cheating on me," etc. were usually solved, often pretty quickly.

Those that were politically motivated took a lot longer. Some never got solved. It depends on the expertise, discipline, skill and goals of the perpetrators, as well as the effectiveness of law enforcement and intelligence organizations.

One thing that did surprise me was the dramatic, wide difference between the number of bombing incidents that actually occur, and the number that make the national news.