Steve Bannon Mark Wilson/Getty Images A group of Democratic senators, including Massachusetts' Elizabeth Warren, want to know whether Steve Bannon, President Donald Trump's chief strategist and former Breitbart News chairman, violated government ethics rules when he communicated with Breitbart about the site's coverage of the White House.

In a Thursday letter to Bannon, the senators pointed to news reports, including one published by Business Insider, that said Bannon instructed Breitbart editors to stop publishing stories critical of chief of staff Reince Priebus.

In February, Axios reported that Bannon was "furious" about the stories concerning Priebus. "Bannon got [Breitbart editor Matt] Boyle on the phone and gave the Washington editor 'both barrels,'" Axios wrote.

Bannon himself confirmed the reporting, telling The Daily Beast that he "went ballistic" during his call with Boyle.

In March, Business Insider reported that Bannon and other White House officials attempted to appease Boyle by giving him one-on-one interviews with Trump, White House press secretary Sean Spicer, and then-deputy chief of staff Katie Walsh.

While Breitbart has denied that Bannon has any influence over their coverage, Alex Marlow, the site's editor-in-chief, told NBC in March that Bannon reaches out to him "every so often."

The letter's authors — Warren, Sheldon Whitehouse of Rhode Island, Edward Markey of Massachusetts, and Mazie Hirono of Hawaii, argued that Bannon's communications with his former employer violate an executive order Trump signed in January. Also known as the White House Ethics Pledge, it prohibits appointees in executive agencies from meeting or communicating with former employers or clients for the first two years of their time in government.

The letter also said Bannon is subject to regulations promulgated by the Office of Government Ethics, which require that executive branch employees "avoid an appearance of loss of impartiality in the performance of ... official duties" by "participating in any matter" with an employer for which the person has worked in the past year.

"Because they relate to news coverage of the Trump Administration, your conversations appear to qualify as ‘communication[s] relating to the performance of [your] official duties with a former employer,'" the senators wrote. "In addition, because you left your position at Breitbart less than one year ago, these communications appear to be in clear violation of both the pledge and OGE regulations."

A similar letter was also sent to Stefan Passantino, the White House's agency ethics official.

The senators demanded that Bannon tell them whether he received any waivers that would have allowed him to communicate with Breitbart employees. They also asked Bannon to reveal whether he has been disciplined by the Office of the White House Counsel.

In March, The Daily Beast reported that no White House employees — all of whom have signed the Ethics Pledge — have received waivers.

Neither the White House nor Breitbart immediately responded to requests for comment.