An airline says an explosion at Istanbul’s Sabiha Gokcen airport has wounded two cleaning staff on a plane, but the cause of the blast was not clear.

Pegasus airline said the blast occurred at 2:05 a.m. Wednesday (7.05 p.m. EST Tuesday) when no passengers were in the area, and that operations were continuing normally.

Turkey’s state-run Anadolu Agency said police armed with rifles and protective vests imposed tight security at entrances to the airport after the blast, searching vehicles in the area.

The agency says one cleaner was seriously hurt, suffering a wound to the head, and the other suffered a wounded hand. Both being treated in a hospital.

The airport operator says the cause of the explosion is under investigation.