Four months after federal investigators started looking into the death of Kendrick Johnson, more people are being called to testify in front of a grand jury in Georgia.

Channel 4 news partner CNN reported that sources with knowledge of the subpoenas say some of the 17-year-old's schoolmates and their parents were called to federal court.

In January 2013, Johnson was found dead in a rolled-up gym mat at Lowndes High School.

The sheriff's office said his death was accidental, and Johnson's parents said that investigation was mishandled.

They believe his death was a homicide.

Sources told CNN that the FBI has interviewed several of Johnson's former schoolmates.

The U.S. attorney handling the investigation said he's satisfied that the FBI is moving at the appropriate speed.