The cause of the crash of the helicopter , which went down on Sunday while on a routine resupply flight between Phnom Penh and Stung Treng , was not known .
<response> The cause of the crash of the helicopter , which went down on Sunday while on a routine resupply flight between <location>Phnom Penh</location> and <location>Stung Treng</location> , was not known . </response>