A U.S. council of regulators looking at whether asset managers pose a potential threat to the financial system said Wednesday that money-market funds can be vulnerable to runs during periods of volatility.

The funds and other firms that lend securities “are vulnerable to same-day calls for liquidity, creating strong pressure to sell assets quickly,” the Financial Stability Oversight Council (FSOC) said in its annual report. Money-market funds “can experience runs when perceived by shareholders to have worrisome risk exposures.”

The FSOC has been studying whether the largest asset managers, such as BlackRock and Fidelity Investments, need Federal Reserve supervision.

Regulators have been calling for additional restrictions on money-market funds since the September 2008 collapse of the $63 billion Reserve Primary Fund.

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Its closure triggered a run on money funds that helped freeze global credit markets.