Shares of Knight Capital Group (NYSE: KCG) were down more than 50% Thursday morning following the previous session’s 33% drubbing after the embattled firm announced a $440 million loss related to a trading glitch that roiled markets.

Knight’s troubles are causing some to wonder if exchange traded funds could be impacted because the firm is a major so-called authorized participant responsible for making markets and providing liquidity in ETFs.

“Knight is a large AP for ETFs,” said Sal Arnuk, co-head of equity trading at Themis Trading, in an email.

Because Knight provides liquidity for so much of the ETF industry, investors should keep an eye on spreads, he said Thursday on Twitter.

“I was wondering out loud what it would mean for ETF liquidity,” Arnuk said.

A Vanguard spokesman said the firm’s ETFs continue to trade normally. “We work with a number of market makers throughout the world, which gives us a certain level of confidence,” he said.

‘Technology issue’

Knight is an authorized participant “for most ETF fund families in the U.S. and across the European landscape,” according to its website.

Some utilities ETFs were indirectly affected Wednesday by Knight’s trading glitch when the funds’ underlying holdings briefly surged in price. [Algos Gone Wild: Utilities ETFs ‘Flash Smash’]