Hundreds of additional stocks and 344 exchange-traded funds face trading curbs under a proposed expansion of the “circuit breakers” that were adopted in response to May’s severe “flash crash,” according to a Reuters report.

The Securities and Exchange Commission (SEC) earlier this month imposed stock-specific circuit breakers, or a mechanism to pause trading, on all stocks in the Standard & Poor’s 500 index.

The SEC on Wednesday proposed expanding the circuit breakers to include all stocks in the Russell 1000 index and a select group of exchange-traded funds, or ETFs, which were hit harder than ordinary stocks in May’s brief market plunge.

A circuit breaker, which halts trading in a stock for five minutes if that stock moves more than 10 percent in five minutes, was launched earlier this month in a pilot program to give the SEC time to make adjustments.

“It is my hope to continue to expand the program to additional publicly traded companies,” SEC Chairman Mary Schapiro said in a statement.

The SEC expansion proposal will be open for a 10-day public comment period after it has been published in the government’s official register. The SEC still has to adopt the rule in order for it to go into effect.

Regulators have yet to determine what caused the Dow Jones industrial average to drop some 700 points in a few minutes on May 6 before sharply rebounding.