Flight recorders showed flight TM470 went down on November 29 while Captain Herminio dos Santos Fernandes manipulated the Embraer 190's autopilot in a way which "denotes a clear intention" to bring the plane down, said Mozambican Civil Aviation Institute (IACM) head Joao Abreu.
"The reason for all these actions is unknown and the investigation continues," said Abreu.
The plane went down in torrential rains in the swamps of Namibia's Bwabwata National Park on November 29, killing its six crew and 27 passengers.
It was flying from the Mozambican capital Maputo to Luanda in Angola.
Abreu told a news conference that Dos Santos Fernandes locked himself inside the cockpit, ignored warning signals and did not allow his co-pilot back in moments before the Embraer 190 hit the ground.
"During these actions you can hear low and high-intensity alarm signals and repeated beating against the door with demands to come into the cockpit," he was quoted as saying by state news agency AIM.
The altitude was manually changed three times from 38 thousand feet to 592 feet -- below ground level -- and the aircraft's speed was also changed manually, according to the preliminary report.
"The plane fell with the pilot alert and the reasons which may have given rise to this behaviour are unknown. At the time, the co-pilot had left the cockpit and was absent while everything happened."