WASHINGTON—U.S. officials concluded there was “no single cause” of the unprecedented volatility that hit U.S. Treasury markets Oct. 15, 2014 instead citing broad changes in the structure of Treasury markets, including the growing role of high-speed trading.

The report, which was highly anticipated on Wall Street, was largely inconclusive on the crucial question of whether regulations adopted since the financial crisis have made markets more prone to swings. It recommended further study of government-bond markets and their participants, along with a review of existing regulations and data released on the market.

The findings represent the latest development in a burgeoning debate about whether financial markets have become more prone to volatility due to new regulations, banks becoming more risk-averse, trading becoming more electronic and nontraditional players taking a growing share of trading volume.

The market for U.S. Treasury bonds, long seen as one of the world’s safest, has shown signs of increasing volatility, particularly on Oct. 15. Trading that day drew comparisons with the 2010 stock-market “flash crash” in which the Dow plunged several hundred points within minutes. Prices of Treasury bonds influence rates on everything from mortgages to car loans and act as a barometer of investor sentiment.

“For such significant volatility and a large round-trip in prices to occur in so short a time, with no obvious catalyst, is unprecedented in the recent history of the Treasury market,” the report said of the events of Oct. 15.