(this article originally appeared in ETF Trends)

As the exchange traded fund universe continues to expand, the Securities and Exchange Commission is preparing for a meticulous review of the industry in response to concerns that the increased interest in ETFs may have exacerbated market volatility.

U.S.-listed ETFs now have $2.4 trillion in assets under management, accounting for 30% of the value of all U.S. shares traded, and the global ETF industry manages $3.3 trillion in assets, the Financial Times reports.

Concerns over ETFs and market volatility heightened following the so-called mini flash crash in August 2015 when more than 1,000 securities were suspended from trading in response to sharp moves, with some ETFs sharply diverging from their net asset values, highlighting the interrelationship between ETFs and the underlying assets.

Consequently, regulators are expected to go through the industry and the consequences of its growth with a fine comb. The SEC could look into the implications of a growing share of the U.S. stock market dictated by ETF flows, along with structural concerns around funds that track bonds.

The SEC has done “bits and pieces,” but is now “laying the groundwork for a bigger review”, a person briefed on the matter told the Financial Times. “ETFs are growing like bunnies. It’s a great success story, but as a forward-looking regulator the SEC has to be on top of any potential issues that may arise in the future.”

The ETF industry’s impact on financial markets may continue to grow as investors increasingly turn to the investment vehicle to gain market exposure. For instance, the underperformance and high fees of traditional mutual funds have accelerated the shift toward passive index-based funds, like ETFs.

Earlier this year, SEC chair Mary Jo White highlighted the ETF industry’s “astounding” growth and hinted the regulator was considering an examination of the potential implications. “The SEC has taken a number of initial actions to share our thinking on these issues, and further regulatory steps beyond additional disclosures may be needed to address some of these issues,” Jo said in May.