Stocks in the U.S. fell sharply after the opening bell, triggering a 15 minute trading halt, and then continued to fall further.

The S&P 500 fell past the 7 percent threshold within minutes of the stock market’s open on Monday, triggering the automatic halt on trading. When trading resumed, stocks continued to head lower.

As of around 10 a.m., the S&P 500 was down 9.9 percent. The Dow Jones Industrial Average had fallen 10.3 percent. The Nasdaq Composite was off by 9.4 percent.

Treasuries rallied, pushing down bond yields. The yield on the benchmark 10-year Treasury note fell to 0.791 percent.

A further decline could trigger another halt. A Level 1 halt, triggered four-minutes after the open Monday, occurs when the S&P 500 drops by 7 percent before 3:25 p.m. Trading is halted for 15 minutes.

A Level 2 halt, which halts trading for an additional 15 minutes, is triggered if stocks fall by an additional 6 percent, a 13 percent overall decline, from the prior close before 3:25 p.m.

A 20 percent decline triggers a Level 3 halt, which stops trading for the remainder of the day.