Illegal “triple witching” trading is being probed.

The Commodity Futures Trading Commission said it is investigating allegations that after-hours trading was conducted in violation of exchange rules during the so-called triple witching hour at the Chicago Mercantile Exchange. The alleged violation occurred Friday, minutes after the close of trading in the Merc’s Standard & Poor’s 500 stock index futures. (The triple witching hour is a quarterly phenomenon when there is simultaneous expiration of stock index futures and options and of options on individual stocks. This sometimes produces volatile trading.)