Mortar rounds caused last month's fatal explosion at Istanbul's Sabiha Gökçen Airport, an investigation has revealed.Istanbul's Anatolia Chief Public Prosecutor's Office said on Thursday that four mortar rounds were fired on December 23 from a forested area around two kilometers [1.2 miles] from the province's Asian-side airport at 02.15 a.m. local time [1215 GMT]. The bi-pod used to stabilize the weapon was found in the area.Out of the four shells hit the apron, three exploded close to each other, while the remaining one fell remotely.Shrapnel pieces of exploding shells hit parking planes and airport staff, the investigation report said.Thirty-year-old airport cleaner Zehra Yamaç died when she suffered a fatal head injury in the blast.Her colleague, Canan Çelik, 33, survived with a hand injury.It remains unclear who carried out the attack or if the assailants have any connection to a terror organization.However, a PKK-affiliated hardliner terrorist group called TAK claimed responsibility for the attack amid ongoing fierce clashes with security forces in Turkey's southeast since July 22.