A joint congressional inquiry is demanding that National Rifle Association executive vice president Wayne LaPierre hand over internal documents showing whether the NRA made “illegal, excessive, and unreported in-kind donations” to the campaigns of Donald Trump and several GOP Senate candidates.

The probe, led by Senator Sheldon Whitehouse, Democrat of Rhode Island, and Representative Jamie Raskin, Democrat of Maryland, is based on a series of investigative reports published by The Trace laying out evidence that the NRA and its vendors used apparent shell companies to evade rules prohibiting coordination between outside groups and the campaigns they support.

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“The evidence shows the NRA is moving money through a complex web of shell organizations to avoid campaign finance rules and boost candidates willing to carry their water,” Whitehouse told The Trace. “And if the NRA can weave such a web, so can Vladimir Putin and others trying to undermine our democracy. We need the truth about this scheme or else special interests like the gun lobby or foreign interests like Russia can flaunt the law and erode the integrity of our elections.”

Whitehouse and Raskin are demanding documents from LaPierre and five related vendors that have either worked for the NRA or for candidates it supported: OnMessage; Red Eagle Media; Starboard Strategic Inc.; American Media & Advocacy Group; and National Media Research, Planning, and Placement.

During the 2014 election cycle, the NRA began steering tens of million of campaign ad dollars through Starboard Strategic, a media strategy firm that appears to exist in name only and works exclusively for the gun group. Since then, the NRA has used Starboard to provide supposedly independent support for a half-dozen Republican senatorial campaigns that were using OnMessage as an ad vendor. The Trace’s reporting, co-published with Politico, demonstrated that Starboard and OnMessage seem to be functionally indistinguishable — they share the same officers and address, and there is no evidence that Starboard has any of its own employees.

Outside groups and campaigns can use common vendors, but the law prohibits their employees from sharing election-related information, like plans for television advertisements or messaging strategy. When that line is crossed, the outside group’s expenditures are no longer independent, and are instead considered direct contributions subject to limits of $5,000 per candidate. Typically, common vendors prevent illegal coordination by making employees sign a “firewall” policy agreeing to follow the law. Despite repeated requests, the NRA and the vendors named in this article have not provided any details about firewall agreements and how they were enforced.

The Trace also reported that the NRA’s ad blitz on behalf of Donald Trump in 2016 was orchestrated by employees of the same firm that the Trump campaign used, resulting in the same pool of strategists buying ads for both Trump and the NRA in the same markets at the same time — seemingly an obvious violation of coordination rules. Federal Communication Commission records show that the NRA spent millions of dollars placing ads through Red Eagle Media, which, according to corporate paperwork, is actually a fictitious business name for National Media Research, Planning and Placement. At the same time, Trump was placing ads through American Media & Advocacy Group (AMAG), a National Media affiliate. The records show that, on numerous occasions, National Media’s chief financial officer, Jon Ferrell, was signing off on placements for Trump as an AMAG employee and for the NRA as a Red Eagle employee.

Among other things, Whitehouse and Raskin are asking the firms and the NRA for details of their firewall policies, documents concerning contracts, and communications between the various named entities. The letters conclude by asking the recipients to retain any information “relevant to this inquiry.”

“When outside spenders coordinate their campaign buys, strategies and messages with political candidates, their expenditures become illegal campaign contributions,” Raskin told The Trace. “Senator Whitehouse and I are trying to determine whether there was a deliberate effort by the NRA and its agents to coordinate with the Trump campaign and other candidates. This matter goes to the integrity of the 2016 election and Congress deserves answers.”

The NRA, OnMessage, and National Media have until March 6 to respond and did not immediately respond to our requests for comment. The letters are below.