Authorities have charged three teenage girls in a high school restroom assault in Delaware that left a 16-year-old girl dead.

The Delaware attorney general's office announced the charges Monday following an investigation into the April 21 death of Amy Joyner-Francis, a sophomore at Wilmington's Howard High School of Technology.

Authorities said a 16-year-old girl, the only person believed to have struck Joyner-Francis, is charged with criminally negligent homicide, and that they will seek to have her tried as an adult. The other two girls are charged as juveniles with conspiracy. The Associated Press is not naming them because they are juveniles.

Authorities revealed Monday that Joyner-Francis died of sudden cardiac death, with a contributing factor of physical and emotional stress due to physical assault.