Several U.S senators on Friday pressed Facebook for answers related to claims the company hired contractors to "retaliate against or spread intentionally inflammatory information about their critics," according to a letter addressed to Facebook CEO Mark Zuckerberg.

"We are gravely concerned by recent reports indicating that your company used contractors to retaliate against or spread intentionally inflammatory information about your critics," the letter reads. "Both elected officials and the general public have rightfully questioned whether Facebook is capable of regulating its own conduct."

The New York Times reported earlier this week that Facebook, in the fallout from Russian interference in the 2016 presidential election, hired Definers Public Affairs to downplay public statements and deflect public scrutiny onto rival tech companies. Definers has been known to conduct opposition research.

The firm reportedly wrote articles that blasted Google and Apple to be published on NTK Network, an affiliate of the firm whose content is often followed by politically conservative outlets, including Breitbart, the report says. In a blog post Thursday morning, Facebook said it ended its relationship with Definers Wednesday night.

Definers issued a statement earlier Friday defending its work with Facebook, though it failed to address claims around NTK Network or that Facebook had ended its partnership.

Facebook's dealings with Definers, if true, "may have campaign finance and other potential legal implications," the senators said.

Senators Amy Klobuchar, D-Minn., Mark Warner, D-Va., Chris Coons, D-Del., and Richard Blumenthal, D-Conn, all signed the letter. Klobuchar was among the senators named in the New York Times report as having engaged in supposedly questionable closed-door dealings with Facebook executives.

Here's the full letter:

Dear Mr. Zuckerberg: We are gravely concerned by recent reports indicating that your company used contractors to retaliate against or spread intentionally inflammatory information about your critics. Since the 2016 election, both the government and your own internal investigations have revealed that your company failed to adequately protect the data of its 2.2 billion users. Your company also failed to implement protocols to prevent manipulation by foreign adversaries working to undermine America's political system. Both elected officials and the general public have rightfully questioned whether Facebook is capable of regulating its own conduct. According to recent reports, your company hired contractors to retaliate and spread intentionally inflammatory information about people who have criticized Facebook, which, if not properly disclosed, may have campaign finance and other potential legal implications. In addition, the staggering amount of data that Facebook has collected on both its users and people who have not subscribed to or consented to use of the platform, raises concern that the company could improperly or illegally use its vast financial and data resources against government officials and critics seeking to protect the public and our democracy. In light of these concerns, we respectfully request you answer the following questions: 1. To your knowledge, did your company hire any entity – including, but not limited to research firms and contractors – to collect or find information to be used in retaliation against people who criticized Facebook, including elected officials who were scrutinizing your company? 2. Did your company hire any entity – including, but not limited to research firms and contractors – to spread negative or intentionally inflammatory information in retaliation against people who criticized Facebook, including elected officials who were scrutinizing your company? 3. Did your company – or any entity affiliated with or hired by your company – ever use any of the vast financial and data resources available to Facebook in retaliation against people who criticized Facebook, including elected officials who were scrutinizing your company? 4. Did your company – or any entity affiliated with or hired by your company – ever seek to conceal information related to foreign interference with the 2016 U.S. election from the public or government investigators? 5. Did your company – or any entity affiliated with or hired by your company — ever contact any media outlets with negative or misleading information, or suggest, promote, or amplify negative or misleading social media about your critics, including elected officials scrutinizing your company? 6. How much money have you expended or paid other entities to collect, find, spread or amplify information about people who have criticized Facebook, including elected officials scrutinizing your company? Has any of that spending been publically disclosed? 7. Some of us have requested that the Deputy Attorney General expand the scope of the Department of Justice's existing investigations to include the latest reports that Facebook hired contractors to retaliate and spread negative information about people who criticized the company. If the Department's investigation is expanded to include this recent report, will you commit to co-operating with any investigation into this matter? Thank you for your prompt attention to this request.

—CNBC's Salvador Rodriguez contributed to this report.