Lawmakers are voicing mounting concerns about a federal tax incentive, known as an “opportunity zone,” that is supposed to encourage investors to pump money into the nation’s poorest neighborhoods.

Leading Democrats in the House and Senate have sent a flurry of letters demanding answers and action by federal agencies after recent New York Times articles detailed how wealthy investors and real estate developers, including those with ties to the Trump administration, are poised to profit on the initiative.

In August, The Times highlighted how tax-advantaged money was beginning to flow to development projects that were underway in affluent neighborhoods even before the opportunity-zone incentive was enacted as part of President Trump’s tax cuts at the end of 2017. The initiative enjoyed broad bipartisan support.

An article last month described how the financier Michael Milken, a longtime friend of Treasury Secretary Steven Mnuchin’s, is among the investors who stand to benefit from the way the Treasury Department is writing the rules governing the tax incentive.