New York state Attorney General Eric Schneiderman's probe of tax practices at private-equity firms is based on information from a whistleblower, according to a person familiar with the matter.

The information came from someone who approached Mr. Schneiderman's office between roughly nine months and a year ago, this person said. Under the state's False Claims Act, the attorney general can investigate alleged fraud against the state based on a whistleblower's allegations.

The...