Trading in all $5.65 trillion of Nasdaq-listed securities was halted for more than three hours on Thursday due to a technical glitch, representing the latest stark example of how complex equity markets have become, Fox Business News reports.  Shares of exchange operator Nasdaq OMX Group Inc. dropped immediately after trading resumed. The Nasdaq Composite Index, however, was up along with other major indexes, reports the Wall Street Journal. The highly unusual flash freeze shutdown did not appear to be caused by a computer hacking incident, according to a federal law enforcement official who spoke to USA Today on condition of anonymity.