BY JEAN EAGLESHAM AND ANDREW ACKERMAN

U.S. securities regulators stepped up their battle against hackers Thursday, filing civil charges against four firms that allegedly became unwitting enablers of a complex fraud by a Latvian trader.

In one of the first cases of its kind, the Securities and Exchange Commission took enforcement action against four U.S.-based electronic trading firms used by the trader as well as charging the alleged hacker himself.

The SEC said Igors Nagaicevs, 34 years old, combined hacking with stock manipulation to execute a “brazen and systematic securities fraud” that netted him more than $850,000 in illegal profits.

Mr. Nagaicevs couldn’t be reached for …