Surging market volatility is making regulators increasingly concerned that bond funds have loaded up on hard-to-sell assets.

The U.S. Securities and Exchange Commission (SEC) has stepped up exams of money managers, while pushing mutual funds to test whether they could satisfy customer redemptions during periods of financial stress, said people with knowledge of the plans. Federal Reserve officials have reached out to the biggest investment firms to quiz them on markets after price swings for stocks, currencies, and commodities hit a 13-month high last week, said a person briefed on the discussions.

“I certainly received a lot of calls from many regulators worldwide over the last few weeks asking me what's going on,” BlackRock Inc. Chief Executive Officer Laurence D. Fink said in an Oct. 21 Bloomberg Television interview. The New York-based firm is the world's biggest money manager with $4.5 trillion of assets.