As the probe of the Indianapolis blast that killed two people enters a third week, investigators continue to grapple with fundamental questions: Who did it? How did they do it? Why? The Indianapolis Star says that despite what is believed -- or assumed -- the answers in such cases are often elusive. Explosions and arson are among the most difficult crimes to solve and prosecute, and can take years to close. Only 19.9 percent of 43,412 arsons ended in arrests in 2011, the FBI says. In Indianapolis, the rate is about the same. 

 "This is a furtive crime and is the hardest to prove," said Peter Beering, an Indianapolis terrorism consultant, former director of the Marion County Emergency Management Agency and a Marion County prosecutor specializing in arson for 15 years. "First you have to prove what happened. Then comes the whodunit part." While investigators pick through debris at the blast site, the investigation is entering the next stage -- the tedious work of analyzing thousands of pieces of evidence, large and small, already taken away to the Marion County and the Bureau of Alcohol Tobacco, Firearms and Explosives crime labs. Detectives are pounding the pavement looking for the person who caused the explosion.