New York attorney general Eric Schneiderman's office sent its first wave of subpoenas to about a half-dozen high-frequency trading firms in an investigation into whether some rapid-fire firms have an unfair advantage over other trading outfits, according to a person familiar with the investigation.

The attorney general is seeking details about whether the firms have secret arrangements with stock exchanges or other trading venues, such as dark pools, that given them the ability to trade ahead of other investors, this person said.