In a statement after the markets closed, Allison Herren Lee, the SEC’s acting chair, and two subordinates said the commission was “actively monitoring the on-going market volatility in the options and equities markets” and was working with other regulators “to assess the situation and review the activities” of those involved.

“Folks at the SEC are going to want to take a very close look at this,” said Harvey Pitt, who chaired the commission from 2001-2003. “When you have extreme volatility of this nature, the system is not necessarily working correctly. … You’re starting to see things that would look to a regulator as if they were manipulation.”

The soaring stock prices for companies that many professional investors scorn illuminated the striking influence of social media, or what Peter Atwater, an economics lecturer at William & Mary, calls a “flash mob with money.”

In their open challenge to the deep-pocketed hedge funds that bet on individual stock prices falling, Reddit’s online mob brandishes an us-against-them rhetoric that echoes the populism of former president Donald Trump’s supporters.