As exchange-traded funds play an ever bigger role in markets and investors' portfolios, a number of recent moves by the Securities and Exchange Commission suggest the regulator is treating these popular investment vehicles with increased skepticism.

ETFs, which resemble open-end mutual funds but trade on an exchange like a stock, have been growing at a rapid clip, with assets of nearly $800 billion and trading volume amounting to roughly a third of U.S. stock trades. But their high profile and increased interest from Main Street mean Washington is watching.

Additional scrutiny could slow the ETF industry's breakneck pace of growth. Exchange-traded funds' basic mechanics have been around for more than a decade. But efforts to tweak and expand the ETF structure beyond one that can accommodate simple bets on stock market indexes has led to more and more baroque designs.

ETFs can now also magnify investors' bets, benefit from the expertise of professional stock pickers or help execute unusual investment strategies such as betting on housing prices or stock market volatility. In the new atmosphere some types of novel and complex funds could be a tougher sell.

In some ways ETFs have always received extra scrutiny. The funds require special legal exemptions in order the operate, and the SEC has always reviewed them before they're marketed to investors. Still, recent steps take it up a notch.

In May, SEC Chairman Mary Schapiro singled out ETFs' widespread trading problems during the May 6 "flash crash" in testimony she gave Congress, noting that about a quarter of all ETFs saw temporary 50%-price declines during the incident.

A preliminary report on the crash issued with the Commodity Futures Trading Commission suggests understanding ETFs' role will be a key focus of the regulators' investigation. However, few lawyers that deal with the SEC like to comment on its policies publicly.

Meanwhile, the SEC also indicated it plans to look carefully before clearing actively managed exchange-traded funds whose designs depart significantly from ones already approved. These proposed funds aim to give fund managers more flexibility by revealing less information about fund holdings to investors, a trade-off that means share prices could be more likely to deviate from underlying values.

When the first active ETFs appeared in 2008, fund companies hoped their success would encourage the SEC to clear progressively more complex versions. Now it's anyone's guess when such funds might materialize.

Finally, the regulator has also showed doubts about so-called leveraged ETFs, which help investors double down or bet against stock- and bond-market indexes. The SEC appeared reluctant when it first cleared these investments in 2006--it had reviewed them for more than four years--and now seems to be having second thoughts.