(Bloomberg News) -- Investment managers facing tough questions from regulators over what caused trading disruptions for hundreds of ETFs said they got hit by a double whammy.

First, volatility hammered futures markets that Monday, hurting liquidity by making it difficult for market makers to obtain information that's vital to valuing ETFs, according to firms such as State Street and Vanguard, which are some of the largest providers in the $2 trillion industry. Then, once buying and selling began later in the morning, prices moved so sharply, they triggered trading halts for more than 500 ETFs.

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