Social media posts, texts, and disturbing drawings are part of the investigation into suspect Ethan Crumbley and his parents, who are also charged.
The department said it "intended to highlight our staff in the Concealed Handgun Permit Office, not to be insensitive."
Rep. Thomas Massie posted the photo days after a shooter opened fire at a high school in Michigan, killing four students.
"The thoughts won't stop. Help me," was written alongside a drawing of a handgun and a person with gunshot wounds, prosecutors said.
In at least one of the notes, police said 37-year-old Katrina Phelan wrote that she was a student who was "tired of being made fun of."
James and Jennifer Crumbley were arrested early Saturday after being declared fugitives. They were charged with four counts each of involuntary manslaughter.
After James and Jennifer Crumbley, who are charged with involuntary manslaughter, were declared "fugitives" after they failed to appear in court.
James and Jennifer Crumbley were charged with involuntary manslaughter in connection to a Michigan school shooting, in which their son is a suspect.
She is charged with one count of false threat of terrorism and one count of using a computer to commit a crime, which each carry a 20-year sentence.
Oakland County Prosecutor Karen MacDonald said the shooting suspect's mother posted about the gun being a gift on social media.
"Unfortunately he was allowed to go back to class," Prosecutor Karen McDonald said. "We now know he had a weapon with him at that time."
Oxford High School, now the site of a mass shooting, was once in the spotlight for a mental health program.
Four students were killed at a shooting in Michigan's Oxford High School. The suspect is a 15-year-old student.
"A journal was recovered from Ethan's backpack also detailing his desire to shoot up the school," Oakland County Sheriff's Lt. Tim Willis said.
Ethan Crumbley, 15, was charged with four counts of first-degree murder in connection with the shootings at Oxford High School.
"All of a sudden, I see 50 people just like running down the hallway, like stumbling over each other, screaming," a 16-year-old student told Insider.
Tate Myre, 16; Hana St. Juliana, 14; and Madisyn Baldwin, 17, were killed on Tuesday after a shooter opened fire at Oxford High School in Michigan.