Heffelfinger, Thomas B.; et al. (Minnesota Department of Education; Minnesota Department of Public Safety; U.S. Attorney's Office, District of Minnesota) (2006). Cooney, Jeanne (ed.). School Safety: Lessons Learned(PDF) (Booklet). United States Attorney's Office, District of Minnesota. pp. 50–2. Archived(PDF) from the original on October 17, 2021. Retrieved March 23, 2021 – via U.S. Department of Justice.
Heffelfinger, Thomas B.; et al. (Minnesota Department of Education; Minnesota Department of Public Safety; U.S. Attorney's Office, District of Minnesota) (2006). Cooney, Jeanne (ed.). School Safety: Lessons Learned(PDF) (Booklet). United States Attorney's Office, District of Minnesota. pp. 50–2. Archived(PDF) from the original on October 17, 2021. Retrieved March 23, 2021 – via U.S. Department of Justice.