Under Democratic leadership, the FCC approved net neutrality rules in 2015, but the agency is now controlled by Republicans and has taken a different stance under the Trump administration. Pai rolled out the FCC's plan to scrap the Obama-era rules before a vote scheduled for Dec. 14.
The review process allows the public to comment on the changes before the agency makes its final decision.
"Enormous numbers of fake comments concerning the possible repeal of net neutrality rules" were first reported in May of this year, Schneiderman's letter said.
The false comments attempted "to drown out and negate the views of the real people, businesses and others who honestly commented on this important issue," it continued.
An FCC spokesperson told ABC News that Schneiderman's "so-called investigation is nothing more than a transparent attempt by a partisan supporter of the Obama administration's heavy-handed internet regulations to gain publicity for himself."
"The potential impersonation of hundreds of thousands of Americans in order to influence the policymaking process should concern everyone — especially the FCC," Schneiderman's office responded. "Yet rather than cooperate with our investigation, the commission has stonewalled it and now offers political attacks to distract from the core issue — the manipulation of the FCC's own regulatory process."
In his letter, Schneiderman wrote that he has "long publicly advocated for strong net neutrality rules" and said his investigation is about "the right to control one's own identity and prevent the corruption of a process designed to solicit the opinion of real people and institutions."
The FCC said it will base its decision on facts and legal arguments, not on repetitive form letters that surface in the commenting process, the spokesperson said. The agency received some 7.5 million comments consisting of the same form letter and over 400,000 comments from an address in Russia in support of the Obama-era rule, according to the spokesperson.
Many of the fake comments "misused the real names and addresses of actual people as part of the effort to undermine the integrity of the comment process," Schneiderman's letter said, adding that "hundreds of thousands of Americans likely were victimized in the same way."
ABC News' Aaron Katersky contributed to this report.