Dennis "D.J." Hernandez -- the older brother of Aaron Hernandez -- has been arrested again ... this time after cops say there was concern he had been planning school shootings at UConn and Brown University.
The disturbing allegations are all spelled out in new Bristol Police Department documents, obtained by TMZ Sports ... where cops say D.J. showed signs this month of being "gravely disabled and a danger to society."
According to the BPD, several people came forward in early July claiming D.