A pilot notified flight attendants that they may encounter turbulence in about 5 minutes, “and suggested they complete their duties within that time,” according to the report.
But within about 4 minutes, the aircraft encountered severe turbulence within the cloud.
A passenger posted on social media that people went flying up from their seats when the aircraft suddenly dropped.
The flight crew was notified of injuries in the cabin and declared an emergency. They requested emergency medical services upon arrival in Atlanta, the NTSB report said.
About 11 people were taken to the hospital for evaluation.
In addition to the four serious injuries, there were eight crew members with minor injuries and five passengers with minor injuries, according to the NTSB. The remaining four crew members and 144 passengers were not injured.
The investigation is ongoing, according to the NTSB.
Delta said it “continues to fully cooperate with the National Transportation Safety Board in their investigation.”