Net sales of exchange-traded funds increased $1.7 billion, according to the monthly report from the Investment Company Institute.

The Nasdaq-100 Index Track Stock, known as its ticker symbol QQQ or Cubes for slang, accounted for a fair share of new money as investors bought $3 billion worth of shares as part of a short-selling strategy, said Avi Nachmany, who leads research at New York-based Strategic Insight. He said this wasn’t the first time the number of QQQ shares went up when the market went down.

Because of a market downturn after the Sept. 11 terrorist attacks, the total value of ETF assets dropped to $64.35 billion last month compared to $72.08 billion in August. Broad-based domestic ETFs dropped in value to $56.2 billion in September from $63.2 billion in August. Sector-based domestic funds declined to $6.1 billion from $6.7 billion. Global/international ETFs fell to $1.9 from nearly $2.1 billion one month earlier.

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