Regulators are ratcheting up their focus on the complex computer systems deployed by high-frequency trading firms, with an eye on whether the systems have adequate safeguards against chaotic trading that can destabilise markets and harm investor confidence.

The Financial Industry Regulatory Authority is conducting a probe of high-speed firms' trading algorithms-the computer formulas that juggle the firms' rapid-fire trades-and the controls surrounding their trading technology, according to an examination letter sent to about 10 firms this week and reviewed by The Wall Street Journal.