Legg Mason Inc. has filed an application with the Securities and Exchange Commission to launch actively managed exchange-traded funds, according to published reports.

The development is another sign of how ETFs are moving from their origins as strictly passive, indexed investments onto the riskier terrain of active management-a trend that is also blurring the lines between ETFs and mutual funds.

Actively managed ETFs were introduced only a few years ago and still make up a miniscule slice of the ETF market. The filing by Legg Mason, however, is an indication of how mainstream investment retailers are getting ready to enter the market.

T. Rowe Price, Putnam, John Hancock Funds LLC and PIMCO are among the companies that have indicated they plan to introduce actively managed ETF products.