The FBI is not a national police force (as Scotland Yard, for example, is), and there are good reasons for this, as any boy who grew up before 1975 knows very well.
The Constitution does not create a general Federal police power. The control of unruly and dangerous individuals in Parkland Florida is the job of the Broward County Sheriff’s Department and the Florida Department of Law Enforcement.
While whoever took this report about this crazy kid should certainly have called the Broward County Sheriff’s Department, and offered technical assistance if requested, it was not their job to find and detain this kid.