Apparently the main plan was Utapao-Gaya-Dushanbe-Zhukovksky, and then the last leg was supposed to be Dushanbe-Zhukovsky non-stop; wich Falcon 10 seems (acc Dassault) the range for that leg its almost unable to do without breaking some rules.
So, running out of fuel, trying to beat headwinds to make it to first Dushanbe, and then to divert apparently to Kulob/Kulyab.
I keep my suspicions if they were indeed found alive, wich we all hope so...