The head of Indonesia’s National Transportation Safety Committee, Soerjanto Tjahjono, says pings detected at sea in the search for a crashed Lion Air plane are definitely from its flight recorder because of their regular pattern.

He said other possibilities have been eliminated because the pings occur at intervals of just under 1 second.

The 2-month-old Boeing 737 MAX 8 plunged into the Java Sea on Monday just 13 minutes after takeoff from Jakarta, killing all 189 people on board.

Tjahjono said a preliminary accident investigation report should be released within a month and the final report could take four to six months.