Captured via Twitter this 28 page report, mainly from their Behavioural Analysis Unit (BAU) and sub-titled:
This report, covering active shooter incidents in the United States between 2000 and 2013, examines specific behaviors that may precede an attack and that might be useful in identifying, assessing, and managing those who may be on a pathway to violence.
Link:https://www.fbi.gov/file-repository/...-2013.pdf/view

It is the second part of a research programme, Part One was published in 2014, which had a different focus.