Last year, The Times reported how money eligible for the tax break — supported by both Democrats and Republicans — was going to luxury projects in affluent neighborhoods, including deals that were underway long before the tax break took effect.

In October, The Times described how the financier Michael Milken stood to benefit from a move the Treasury Department made over the objections of some agency officials to permit a census tract in Nevada to qualify for the Opportunity Zone tax break. Mr. Milken is a longtime friend of Treasury Secretary Steven Mnuchin’s.

“Despite these warnings from staff, Secretary Mnuchin instructed Treasury officials to allow the otherwise ineligible tract to qualify for the incentive,” the lawmakers wrote in seeking the inquiry. “If the Treasury Department provided a stamp of approval as a political favor, it is not only unacceptable, but in complete violation of the congressional intent of the Opportunity Zones.”

The Treasury’s internal watchdog expects “to complete our work and respond to the congressional requesters in early spring,” Rich Delmar, the department’s deputy inspector general, said in a statement.

Other potential beneficiaries of the Opportunity Zone tax break, The Times reported last year, were billionaire financiers like Leon Cooperman; Chris Christie, the former New Jersey governor; Richard LeFrak, a New York real estate titan who is close to the president; and the family of Jared Kushner, Mr. Trump’s son-in-law and senior adviser.