(CNN) The Treasury Department's inspector general has opened an inquiry into a controversial tax break used by close allies of President Donald Trump.

The review into the agency's Opportunity Zone program is in response to a request from three Democratic lawmakers, said Acting Treasury Inspector General Richard Delmar.

The program, created by the 2017 GOP tax bill, allows investors to defer taxes on capital gains if they're reinvested in a real estate project or business located in one of nearly 9,000 qualified census tracts. Any capital gains from those investments will be untaxed if they're held for more than 10 years, a tax giveaway worth $7.7 billion through 2022, according to the Joint Committee on Taxation.

The company and associated entities, which already owned properties in a New Jersey area that was designated this year as an opportunity zone, had at the time spent more than $13 million buying new properties that would be eligible for the tax break, according to Monmouth County public records.

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