According to the New York Times:
The breadth of [the Russian authorities' possible-cause] list underscored that no clear causes had emerged, leaving open the possibility that the crashes had been an awful coincidence - a case of two jetliners leaving the same terminal at Domodedovo International Airport and having catastrophic accidents minutes apart.I can see how that would be plausible, since there are something like 430 million annual commercial airline flights worldwide, and a couple of dozen of them crash every year. When one in ten million flights crash, its inevitable that you'd get a couple of them clustered together at the same location within the same four minutes of each other.
According the the AP:
But despite the timing and circumstances of the crashes, officials say no firm evidence of terrorism has yet been found in the planes’ charred wreckage and they’re looking into the possibilities of poor fuel and human error."Firm evidence" being a box of business cards from Terrorists R Us, evidently.