Rossi mentions "going postal." In 1986, a postal worker in an Edmond, Oklahoma, post office killed fourteen people, wounding several others, before committing suicide. The term "going postal" began being used later after more mass shootings followed by other postal workers.
Based on the date stamp on one of the victims' cellphone selfies, the date of the execution is April 28.
The tattoo on the unsub's wrist is the image from the Oklahoma flag.
While Oklahoma does have firing squads as a method of execution, no inmate has been executed via that method. 82 were executed by electrocution and one by hanging from 1912-1966. 112 were executed by lethal injection from 1990 to 2015. In 2015 Oklahoma put in place a moratorium on executions following botched executions in 2014 and 2015. Oklahoma is one of three states that allow execution by firing squad, those being Mississippi, Oklahoma, and Utah.
Casey Sander (Detective Childers) also played Mike Rostenkowski on The Big Bang Theory (2007), who was Bernadette's father, a police officer. On the same show, Penny's dad is played by Keith Carradine, who played Frank Breitkopf - Gideon's main antagonist.