Sen. Elizabeth Warren (D-MA) takes the stage during the first night of the Democratic presidential debate on June 26, 2019 in Miami, Florida.

"Unfortunately, it appears as though the agency itself may have misled the American public about the terms of the Equifax settlement and their ability to obtain the full reimbursement to which they are entitled," she wrote.

"The FTC has the authority to investigate and protect the public from unfair or deceptive acts or practices, including deceptive advertising," Warren wrote to Andrew Katsaros, the agency watchdog, in a letter dated Tuesday.

Presidential contender Sen. Elizabeth Warren wants the Federal Trade Commission's inspector general to open an investigation into the agency after it announced that victims of the Equifax data breach will get "nowhere near" the $125 compensation package originally advertised.

The FTC announced last month that Equifax agreed to pay up to $700 million to settle a range of government claims against it for the 2017 breach that exposed the personal information of nearly 1 in 2 Americans.

The settlement required the company to provide free credit monitoring to every individual affected by the breach, or compensation of "up to" $125 each for those who opted out. To fund the compensation package, the settlement created a $31 million fund.

Warren notes that fund "would pay exactly 248,000 individuals" the full $125, "less than 1% of the 145 million individuals affected by the breach."

According to the FTC, millions of people visited the settlement website in just the first week.

Under the terms of the settlement, if more than 248,000 people sought compensation, the amount paid to each individual would be reduced.

"There's apparently a run on settlements so there's anxiety people are going to get 16 cent checks," Rep. Alexandria Ocasio-Cortez, D-N.Y., wrote in a July 26 post on Twitter, warning her followers to select the free credit monitoring. The New York congresswoman had originally advised her followers to "go get your check from Equifax! $125 is a nice chunk of change."

AOC tweet

Warren wrote that the FTC seemed to mislead customers about the possibility that payments could be less than $125. On at least two occasions, she wrote, the FTC "appeared to misinform consumers by failing to tell them about these potential reductions until after they had signed up for payments."

In one statement, published the day the settlement was announced, the FTC claimed that Equifax victims could claim "free credit monitoring OR $125 if you decide not to enroll." In an infographic that was on the FTC website until at least Aug. 6, the FTC said consumers could ''sign up for free credit monitoring for up to 10 years OR get a cash payment of $125 for credit monitoring you already have."

"These pages did not inform consumers that the cash payment was subject to — and in fact, was very likely to be — severely reduced," Warren wrote. "As of today, the infographic on the FTC website now does not even mention the possibility of consumers receiving the $125 payment."

Warren's letter asks for the inspector general to investigate whether any "individuals or offices" at the FTC violated a federal law that prohibits "unfair or deceptive acts." The FTC is the agency responsible for overseeing the law.

The Massachusetts senator is also asking for an investigation into whether the FTC had "any estimate" of how many people would choose the cash payment option, and how the agency arrived at that estimate.

Equifax and the FTC did not respond to requests for comment. On its website, the FTC notes that it will be possible for those who have claimed the cash option to switch to free credit monitoring instead. Those who have submitted a claim for cash payment will receive an email from the settlement administrator with more details, the website says.

The letter comes months before the filing deadline to submit claims under the settlement, which is Jan. 22, 2020. Benefits will be delivered "Jan. 23, 2020, at the earliest," according to the FTC.

— CNBC's Jordan McDonald contributed to this report.