Four Democratic senators joined a growing chorus of Facebook critics on Friday, sending a letter to the company’s leadership requesting answers about its employment of a Washington public relations firm.

The four senators — Amy Klobuchar of Minnesota, Mark Warner of Virginia, Chris Coons of Delaware and Richard Blumenthal of Connecticut — asked Facebook CEO Mark Zuckerberg in the letter whether his company had hired a firm to collect and disseminate information about Facebook critics, including elected officials.

“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 senators wrote.

The letter adds to what has become Facebook’s latest crisis over its handling of Russia’s efforts to spread disinformation and divisive political rhetoric on the company’s social network.

On Wednesday, The New York Times published an in-depth look at the company’s reaction to the Russia scandal, including efforts to downplay the severity of Russia’s actions and hit back at critics — in particular by hiring Definers Public Affairs, which the Times found pushed information connecting Facebook critics to billionaire philanthropist George Soros and ran what one employee called an “in-house fake news shop.”

The Times article noted that Zuckerberg and Sheryl Sandberg, Facebook’s chief operating officer, were “bent on growth” and that “the pair ignored warning signs and then sought to conceal them from public view.”

Citing the company’s role in failing to “implement protocols to prevent manipulation by foreign adversaries,” the senators noted that this was not the first time the company’s actions led to larger questions about the need for regulation.

“Both elected officials and the general public have rightfully questioned whether Facebook is capable of regulating its own conduct,” the letter read.

Facebook COO Sheryl Sandberg testifies during a Senate Intelligence Committee on Capitol Hill on Sept. 5, 2018. Drew Angerer / Getty Images file

On Thursday, the same group of senators submitted a letter to the Department of Justice, urging it to investigate the allegations that Facebook hired a firm to retaliate against critics.

Zuckerberg and Sandberg have each issued public statements acknowledging the company’s shortcomings in responding to Russia’s actions but pushed back against claims that it had attempted to hide the problem.

“Mark and I have said many times we were too slow,” Sandberg wrote in a Facebook post on Thursday night. “But to suggest that we weren’t interested in knowing the truth, or we wanted to hide what we knew, or that we tried to prevent investigations, is simply untrue.”

According to the Times, one of the groups Definers tried to smear was the racial justice organization Color of Change, pressing reporters to question its ties to Soros.

“By suggesting to reporters that Color Of Change is acting as the puppet of Mr. Soros merely because he is one of our many funders, they have given oxygen to the worst anti-Semitic conspiracy theories of the white nationalist Trump base,” Rashad Robinson, president of Color of Change, said in a statement on Thursday.

Both Zuckerberg and Sandberg have publicly denied that either knew of the company’s history with Definers. On Thursday, Facebook terminated the company’s relationship with the firm.

Tim Miller, a partner at Definers, sought to clarify the firm’s actions, writing in a blog post on Friday that the firm was trying to illustrate that Facebook opposition had not been a grassroots operation. Miller said Definers found that Freedom From Facebook, one of the organizations that Facebook reportedly sought to target, was backed by a variety of organizations, including the nonprofit organization the Open Markets Institute, which is funded by Soros.

“Regardless of whether you think it’s a good or bad thing, this funding is potential news for a reporter who covers tech policy to explore,” Miller wrote.

But the momentum against Facebook continued to mount, as organizations and activists called for the company to face federal regulation.

Freedom From Facebook filed a complaint Thursday to the Federal Trade Commission asking the agency to investigate a data breach that had affected close to 30 million users in September and to enforce any violations Facebook may have made to a FTC’s 2011 consent decree. Facebook could face financial penalties if it is found to have violated the FTC order.

“Facebook, Inc. is a serial privacy violator that cannot be trusted,” the complaint stated. “It has grown too big and its products have become too integrated and too complex to manage. Not only can we no longer trust Facebook, Inc. to manage its system safely, the corporation no longer has the capacity to do so effectively.”

Facebook did not immediately respond to requests for comment about the FTC complaint or the senators’ letter.

Some lawmakers say they are just as frustrated with what they feel is a lack of federal regulation and enforcement of Facebook — particularly Democrats who will be better positioned to crack down on social media platforms when they take control of the House in January.

I agree with ⁦@RepCicilline⁩. We simply cannot rely on Facebook and other social media companies to regulate themselves. Hate is spreading & being given oxygen on these platforms--their profit model relies on this. We need to address this quickly. https://t.co/GsJO14wLjU — Rep. Pramila Jayapal (@RepJayapal) November 16, 2018

“We simply cannot rely on Facebook and other social media companies to regulate themselves,” Rep. Pramila Jayapal, D-Wash., tweeted Friday morning. “Hate is spreading & being given oxygen on these platforms — their profit model relies on this. We need to address this quickly.”

Both organizations and lawmakers have also called on Facebook to enact additional safeguards against itself, such as appointing a company ombudsman.

“An independent board chair is essential to moving Facebook forward from this mess, and to re-establish trust with Americans and investors alike,” Scott Stringer, New York City comptroller, said in a statement.

Strong government action on the company may not come easily. Rep. David Cicilline, D-R.I., on Thursday spoke about the need to weaken Facebook’s “connection” to Washington’s power centers.

“When you have the kind of economic power and the concentration of economic power that you see in a company like Facebook, it’s very often accompanied by strong political power,” Cicilline said in an appearance on MSNBC. “And so we’ve got to break that connection.”