They were running a drill.
This was pointed out before. Something about a drill. FWIW there has always been a security force for the Boston Marathon since 2001. Both at the gate and finish lines. Homeland security, FBI and state and local police are always in attendance. I haven’t watched the marathon live for about 5 years but the last time I was in the finish line area (my wife was running) the added security was quite noticable. If it was increased beyond what I saw back then something must have been up.
what a coincidence. just like sandy hook. drill was 3 miles away on dame day.