The Republican National Committee and the Trump 2020 campaign wrote to Facebook and Twitter Thursday requesting answers to questions about their efforts to prevent political bias on their platforms.

The letter comes amid concerns about the censorship of conservative speech online.

The letter, signed by RNC Chairwoman Ronna McDaniel and Trump campaign manager Brad Parscale, is addressed to Facebook CEO Mark Zuckerberg and Twitter CEO Jack Dorsey and expresses alarm at allegations that the sites have blocked conservative content and users.

“We recognize that Facebook and Twitter operate in liberal corporate cultures. However, rampant political bias is inappropriate for a widely used public forum,” the letter says. 

“What’s more, the consequences to our democratic society are profound. That is why special attention is necessary to address issues such as Facebook’s use of liberal organizations to fact-check content, the access to user profiles it gives to some employees, and Twitter’s manipulation of content that appears for the site’s users.”

Zuckerberg defended his site in front of a hearing on Capitol Hill last month, and last week in the European Parliament -- while addressing concerns about bias as well as security and privacy. In D.C., he admitted that such concerns were understandable as Silicon Valley “is an extremely left-leaning place."

"This is actually a concern that I have and that I try to root out in the company is making sure that we don't have any bias in the work that we do," he told Sen. Ted Cruz, R-Texas. 

The letter cites voices such as pro-Trump personalities Diamond and Silk, who say they have been blocked by Facebook, and calls for the giants to be transparent about what they are doing. It also says that Facebook is working to encourage voter registration, but expresses concerns that “the new feature does not become essentially an in-kind contribution to liberal candidates.”

Facebook did not respond for a request for comment by Fox News, while Twitter declined to comment.

In his first TV interview since becoming Trump’s campaign manager, Parscale said on Fox & Friends that he receives thousands of messages from people saying they have been censored.

“We want to ask them for transparency, I think the public deserves that transparency and we need to know that conservative voices have the chance to get their message out,” he said. “This is a big problem.”

He said that on Facebook, algorithm changes have led to a decrease in reach by as much as 40 percent.

“We need to make sure that conservative ideas and principles are allowed on these social-media platforms and there’s not a thought police coming from Silicon Valley,” McDaniel said.