God please. I'm already seeing "Oh look, coincidental bomb squad drills" accusations. One guy claimed bomb squad people were at the finish line telling people they were drilling and to be calm.
IF any of this is true, it points to previous threat and a desire to keep it quiet yet investigate it to avoid upsetting people, not that it's a "false flag."
If they're going to bomb themselves, why would a bomb squad be waiting with coffee and donuts for it to go off? What on earth for?