What is up with the shirts, you ask?
There are a dozen issues that raise questions. My first guess would be that the shirts were camouflage (so when the back pack is first opened there would be no exposed wires). They might have also been tied to hold the charge in the proper orientation.
A smart bomber would have used very common items (not limited edition t-shirts related to local events), that were not his personal things.
"The family that puts the needs of the family above the whims of its children will prevail over the family that does not."