Oil has lost about half its value in the past few months, so does this vindicate Opec's claim that speculators drove it upwards?

Oil has lost around half its value since peaking at record levels earlier this year and Opec has blamed speculators for driving the price. Is the oil organisation justified in claiming it was speculation, and not price manipulation, that drove the price? Are these speculators now also to blame for the rapid decline in the price of crude?