It was a new first stage.
The previously flown stage was scheduled for a flight in mid October.
mheslep said:
it suggests a possibly pathological problem with safety.
Does it? Unmanned rockets typically have a 5% loss rate (counting from a lauch attempt). SpaceX lost one Falcon 9 out of 28. Even if we include this mission (then we should also change the 5% but I don't know the number then), we have 2 out of 29 - above the average, but not significantly. If those missions would have been manned, the crew would have survived in both cases - in the first case the Dragon capsule survived the explosion even without an abort system, and in the second case the crew would not have been on board.
There were 312 manned launch attempts so far, two of them failed, the launch escape system rescued the crew in one of them (a Russian rocket that exploded on the launch pad). That makes one loss out of 312 crews - higher than what SpaceX would have delivered, although the difference is not significant of course.
There were also three landing attempts that ended fatally - one Space Shuttle, one Soyuz that crashed into the ground, and one Soyuz that lost its interior pressure while still in space. The last one killed 3 astronauts, to the only deaths in space so far.
Of course they will... of course they
are investigating the explosions already to figure out what went wrong. Last time they had a good idea after just 2 days, and could reproduce it in their test stands after 2 days more. Then they spend several months improving everything and checking it over and over again.