There are going to be any number of theories advanced as this tragedy is forensically analyzed, one of the main questions being - was this an act of domestic or international terrorism?
My gut feeling as an old soldier is that it was the work of amateurs. A professional would have timed the first explosion when there were more people near the finish line, and the second explosion to detonate in the same vicinity, minutes after the first. That would have taken out the first responders who rushed in to aid victims of the first.
During the fighting in Sarajevo, a group of teens were congregating outside the Canadian compound on a Sunday morning. A mortar bomb landed in the middle of them and Canadian soldiers immediately responded, against protocol for the above mentioned reason.
The Canadian CO, Major-General Louis Mackenzie was one of them. Despite the risk of a second incoming bomb, they felt it was the 'right thing to do' and they responded automatically. Whoever fired the mortar, probably didn't want to kill or injure members of the ISAF, but the Canadians didn't know that.
After returning to Canada, Maj.-Gen Mackenzie suffered a PTSD reaction. While picking up his Sunday morning paper from his doorstep, he saw a group of Canadian teens on the street and it triggered the memory. His wife found him sobbing on the doorstep. He went public with it, opening the door for his soldiers to get needed counselling following their deployment.
God comfort those who grieve and mourn and God bring those who commit such crimes to justice. Amen.