Nine U.S. senators are calling for a Federal Election Commission investigation into the National Rifle Association’s use of an apparent shell company to skirt election laws.

In a joint letter to FEC Chair Caroline Hunter and Vice Chair Ellen Weintraub, the lawmakers — led by Senator Sheldon Whitehouse of Rhode Island — asked the FEC to “open an investigation into a potential campaign finance violation” alleged by the Campaign Legal Center, a watchdog group, in two complaints that are currently before the commission.

The complaints claim that the NRA uses a company called Starboard Strategic Inc. to circumvent laws prohibiting election-related coordination between campaigns and outside groups who support them. They grew out of reports published by The Trace over the summer showing that Starboard — which exclusively services the NRA, and has received more than $60 million from the gun group — appears to be functionally indistinguishable from OnMessage Inc., a prominent political consulting firm. Corporate records show that Starboard and OnMessage share the same principals and offices.

When campaigns and outside groups use a common vendor, they are required to establish a “firewall” to prevent executives and staffers working for either client from sharing information. OnMessage and Starboard have repeatedly refused to share details of any such firewall policy.

Prior to the creation of Starboard in 2013, the NRA used OnMessage as a vendor to place political ads. Beginning in the 2014 election cycle, the group shifted to Starboard, spending millions of dollars for ads supporting the campaigns of three Republican Senate candidates: Thom Tillis in North Carolina, Cory Gardner in Colorado, and Tom Cotton in Arkansas. All three campaigns paid OnMessage as a consultant, and all three won.

In 2016, a similar arrangement played out in Wisconsin: Ron Johnson, defending his Senate seat against Democrat Russ Feingold, hired OnMessage. Later, the NRA paid Starboard for ads attacking Feingold. Johnson won his race.

The NRA buys ads through Starboard Strategic, which has no other clients. Its favored candidates use OnMessage. But the two firms appear to be identical.

In the current cycle, two Republican Senate candidates — Matt Rosendale in Montana and Josh Hawley in Missouri — have enlisted the services of OnMessage. Over the last month, the NRA has paid Starboard more than a million dollars for ads bolstering their campaigns.

“Right-wing and corporate special interest money flooding our elections is corrupting our politics, and chipping away at Americans’ trust in our government,” Whitehouse told The Trace. “We need to ensure that the FEC is doing its part to enforce existing campaign finance laws. That’s why my colleagues and I are calling out what appears to be a clear violation of anti-coordination rules.”

The letter — also signed by Dianne Feinstein, Elizabeth Warren, Patty Murray, Richard Blumenthal, Christopher Murphy, Ed Markey, Chris Van Hollen, and Kamala Harris — says that it is “highly likely” that OnMessage and Starboard broke the law by “sharing proprietary information related to candidates and campaign expenditures.”