Macon, Georgia (CNN) -- Kendrick Johnson's former schoolmates and their parents have been subpoenaed by a federal grand jury investigating the Georgia teen's 2013 death, according to sources with knowledge of the subpoenas.
Among those seen entering the federal courthouse in Macon on Thursday were current students at Lowndes High School and current and former students at Valdosta High School, also located in Lowndes County.
Thursday's development comes four months after the feds launched an investigation. Johnson's parents have said the original probe was mishandled by local authorities. The FBI has interviewed several of Johnson's former schoolmates, according to a source with knowledge of the investigation.
Outside the federal courthouse Thursday, U.S. Attorney Michael Moore said, "We're working methodically, and sometimes we remember we're running a marathon instead of a sprint. So, we're working on it. It's better to get it right than to get it fast. I'm satisfied that the FBI is moving forward at the appropriate speed, and they're doing a fine job."
Johnson's body was found in a rolled-up mat in the Lowndes High School gymnasium on January 11, 2013. Investigators with the Lowndes County Sheriff's Office ruled his death accidental, concluding that Johnson climbed into the center of the gym mat to reach for a shoe and got stuck.
State officials later concurred that Johnson's death was accidental and listed his cause of death as "positional asphyxia," or suffocation by his own body weight.
Johnson's parents have called the official story a "coverup" and said they believe their son's death was a homicide. A separate autopsy commissioned by the family attributed the 17-year-old's death to "apparent non-accidental, blunt force trauma."
The complete list of those subpoenaed for Thursday's grand jury proceeding was not clear, but Lowndes County Sheriff Chris Prine said he had not received a federal subpoena, and Lowndes County Schools said it was subpoenaed only to provide hard drives containing surveillance video from the school.
County Coroner Bill Watson said he hasn't received a subpoena, either, though the FBI has interviewed him twice and he has spoken to the U.S. attorney. Johnson's parents say they, too, spoke with the FBI during a three-hour interview in Thomasville, 40 miles west of Valdosta, in December.
Asked during a January interview if he found the feds' involvement encouraging, Johnson's father, Kenneth Johnson, said, "We really won't feel anything until justice comes for Kendrick. We're glad they're here, but my feelings won't be there until we get justice for our son."
His wife, Jacquelyn Johnson, said the FBI interview left her hopeful.
"They answered a lot of questions about a lot of what we had heard," she said. "I felt better because it's another set of eyes looking into the case."