Why is the Austin bomber so hard to catch?

It appears that Tuesday’s explosion at a FedEx sorting facility is linked to the previous four explosions that have afflicted Austin, Texas since March 2. If so, that means the bomber has five explosions under his or her belt in under three weeks, with more expected to follow.

So why is the Austin bomber proving so hard to capture? After all, considering the vast resources of the federal government; including hundreds of federal agents acting with counter-terrorism investigative authorities, many observers might have expected the culprit to have been found by now.