Fourth, Homeland Security investigators will examine who flew out of Boston, especially to foreign locations, around the time of the bombing. In the 1993 World Trade Center attack and the CIA Headquarters-Route 123 shootings, attackers moved quickly to the airport to get out of the country.
Fifth, police and federal agents will contact all of their informants in Islamist cells and American right wing and Aryan supremacist groups. They will offer rewards for leads, for suspicions. Unless the attack was done by a self-initiated loner, which is highly possible, the bomber or bombers will tell someone eventually of their involvement. Alternatively, as in the case of the Unibomber, a family member may call the police when they realize that their brother, husband, or son may have been involved.
In major investigations such as these, the case is usually solved either very quickly due to a lucky break or it takes months of painful detailed work. Sometimes, as in the Atlanta Olympic park bombing and the TWA 800 crash, the investigation may go down a mistaken path for a time. However long it takes, these cases do get solved.
For me, a Bostonian who grew up watching the end of so many Marathons, spent so many days hanging out in Copley Square, this attack feels personal. The eight year old boy who died was from the Dorchester neighborhood, "Dot," where I lived when I was an eight-year-old boy. For that boy and the other victims, this case will be solved and the criminal terrorist will be apprehended.
Richard Clarke is a former White House counter-terrorism adviser and current ABC News consultant.