Story highlights Original investigation was "timely and properly prioritized and handled," investigators say

Georgia teen was found dead, rolled in a gym mat, nearly a year ago

Lowndes County Sheriff's Office ruled the death an accident last year

Johnson's parents think there's more to the case; a federal investigation is in progress

Kenneth and Jacquelyn Johnson still spend their days outside the Lowndes County Courthouse in Georgia, standing vigil for their son, Kendrick, whose mysterious death a year ago poses many questions in their minds.

They simply want to know how their son ended up in a rolled-up mat in his high school's gymnasium on January 10, 2013; his body was found a day later, when students in a gym class glimpsed his feet in the mat.

The original investigation by the Lowndes County Sheriff's Office concluded that 17-year-old Kendrick Johnson died by accident while reaching for a shoe, but the Johnsons don't believe that. They're planning yet another rally in Valdosta's James W. Saunders Memorial Park at noon Saturday to again demand answers.

"They know something happened in that gym and they don't want it to come out," Kenneth Johnson said.

Federal authorities, too, have their doubts about the original investigation's findings. CNN has learned that the FBI has been in South Georgia conducting interviews, and the Johnsons recently spent almost three hours with investigators at an FBI field office in Thomasville, an hour west of their hometown.

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U.S. Attorney for the Middle District of Georgia Michael Moore said in November that federal authorities would investigate Johnson's death , and last month, the FBI seized the original hard drives from the surveillance system at Lowndes High School, according to a source with knowledge of the investigation surrounding the hard drives.

The federal investigation has yielded few details, but Moore issued a statement Thursday: "As we approach the anniversary of Kendrick's death, I am mindful that there is a great deal of interest in the status of our efforts. As I have previously stated, in order to protect the integrity of any ongoing investigation by my office, I typically refrain from making any specific comments about the investigation until it is concluded. As we continue our work in this matter, my thoughts and prayers will be with the Johnson family this weekend."

The Lowndes County Sheriff's Office hasn't commented on every twist and turn in the case but has repeatedly stood by its investigation.

"It was not a rash decision that was leapt to. We've drawn this out, done a thorough investigation, and we think we've covered all bases," Lt. Stryde Jones told CNN in May, the day after investigators closed the case.

On Friday, the office released a statement through its attorney, saying, "We continue to support Mr. Moore's interest in these tragic circumstances and applaud his urging of the community to come forward with facts and evidence rather than feelings or opinions, no matter how sincere they may be."

The Sheriff's Office said it had interviewed more than 100 people -- students, teachers and parents among them -- but the timing of those interviews raises questions for at least one analyst.

CNN examined the 522-page police file and found that investigators spoke to 111 people. Eighteen of those interviews occurred the day Johnson's body was found, and another 15 people were interviewed later that month. Investigators then spoke to five more people in February and March before conducting interviews with 72 people in April, about the time media coverage of Johnson's death gained momentum. One person was interviewed in May, according to the investigative file.

Former Washington Metropolitan Police Department Detective Mike Brooks, an analyst for CNN's sister network, HLN, said that if he were involved in the investigation, he would "want to interview someone that day, that night or maybe the following day while things are still fresh in their mind."

The first responders weren't interviewed until April 17, according to the file, and the paramedic who noticed bruising on Johnson's jaw -- and considered the gym to be a crime scene -- wasn't interviewed until April 18. Investigators didn't speak to the janitors who cleaned the gym until April 26.

Asked why the bulk of the interviews weren't conducted until April, the Sheriff's Office said, in the statement released by its attorney, that "Sheriff (Chris) Prine maintains every confidence that his office's investigation was timely and properly prioritized and handled with the necessary diligence to assure that all leads were examined and exhausted and he welcomes the U.S. Attorney's continued review of the case."

Surveillance video shows other students in the gym with Johnson before he died, but according to local investigators' reports, there's no record of interviews with any of them, although they spoke to more than 90 students.

"I looked at the crime scene investigation, evidence collection, the autopsy, the interviews that were done and were not done in a timely fashion. This is a sloppy case from the get-go," Brooks said.

The Sheriff's Office noted that it provided its investigative files to CNN in May, but the names in that file were redacted. CNN did not receive the unamended version until October 30, and the network did not receive the surveillance video until November 6.