U.S. CFTC head - market complexity a factor in time taken to charge trader

CHICAGO (Reuters) - U.S. financial markets' size and complexity were a factor in how long it took to bring charges against a British trader accused of contributing to Wall Street's "flash crash" nearly five years ago, the head of the U.S. Commodity Futures Trading Commission said on Wednesday. "Sometimes it takes a long time to put cases together," CFTC Chairman Tim Massad said in response to questions following a speech he delivered at a derivatives industry conference in Chicago. "These are huge markets," Massad said. "There's a lot going on." On Tuesday, the CFTC filed civil charges against Navinder Singh Sarao who was arrested in London. His arrest followed criminal charges brought by the U.S. Justice Department for wire fraud, commodities fraud and manipulation. (Reporting by Tom Polansek and Karl Plume in Chicago, Writing by Dan Burns in New York; Editing by Douwe Miedema in Washington and Christian Plumb)