Screenshot by Sean Hollister/CNET

Lawmakers are asking the tech industry's heavy hitters to ban online distribution of designs for 3D-printed guns.

Five Democratic senators on Tuesday sent letters to Twitter, Facebook, Microsoft, Craigslist, Google, Reddit and Yahoo asking the companies to block blueprints for 3D-printed guns on their platforms.

They're worried because anyone with a computer could download design files for gun parts and use a 3D printer to manufacture them, with two consequences: Authorities would have no record of those guns, and the materials used in the printing could make them hard to spot with scanners.

In July, the State Department settled a legal case and let an organization called Defense Distributed release gun plans online. The move prompted a lawsuit from several state attorneys general, and a Seattle judge granted a temporary restraining order to block the publication of blueprints until a hearing set for next week.

"The publication of these CAD files would therefore make available to any person deadly weapons that are untraceable and undetectable, thereby undermining firearms laws and posing a serious risk to the safety of our communities," the senators wrote in the letter.

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Last week, Facebook banned sites that host blueprints of 3D-printed guns from its platform. It's unclear whether other tech companies will do the same.

"Sharing material on how to manufacture firearms using 3D printers or CNC milling machines is not allowed under our Community Standards," a Facebook spokesperson said in an email statement. "In line with our policies, we are removing this content from Facebook."

The five senators -- Edward Markey, Dianne Feinstein, Bill Nelson, Robert Menendez and Richard Blumenthal -- didn't immediately respond to requests for comment.

"We are constantly reviewing our policies to ensure that they keep pace with new technological realities," a Reddit spokesperson said in an email statement. "As of August 15, Reddit's site-wide policies explicitly prohibit the online dissemination of 3D printed gun design files."

Twitter declined to comment. Microsoft, Craigslist, Google and Yahoo also didn't immediately respond.

Though a few people familiar with the matter told CNET that they haven't see much of the design files on their platforms.

Here the full text of the letter sent to Twitter CEO Jack Dorsey:

Dear Mr. Dorsey: We are writing to urge Twitter and other prominent Internet search and social media companies to take immediate steps to block or limit access to 3-D gun printing materials available online. Recently, a company called Defense Distributed announced that it would publish over the internet the blueprints for 3-D printable firearms, in the form of Computer Aided Design (CAD) files. The CAD files would allow users from all over the world to automate production of a gun using a 3-D printer. The publication of these CAD files would therefore make available to any person deadly weapons that are untraceable and undetectable, thereby undermining firearms laws and posing a serious risk to the safety of our communities. While a district court recently issued a temporary restraining order to prevent the online publication of these CAD files, the order was just temporary. Moreover, there are already reports that thousands of such CAD files have already been downloaded by individuals, who can now disseminate the files privately or publicly on a variety of forums. That is deeply troubling. We appreciate that your company, like other social media companies, has terms of service that allow Twitter to remove content like 3-D gun designs from its platforms: Terms of Service: "We reserve the right to remove Content that violates the User Agreement, including for example, copyright or trademark violations, impersonation, unlawful conduct, or harassment." Twitter Rules: "All individuals accessing or using Twitter's services must adhere to the policies set forth in the Twitter Rules. Failure to do so may result in Twitter taking one or more of the following enforcement actions: · requiring you to delete prohibited content before you can again create new posts and interact with other Twitter users; · temporarily limiting your ability to create posts or interact with other Twitter users; · asking you to verify account ownership with a phone number or email address; or · permanently suspending your account(s)." "You may not use our service for any unlawful purposes or in furtherance of illegal activities. By using Twitter, you agree to comply with all applicable laws governing your online conduct and content." "You may not make specific threats of violence or wish for the serious physical harm, death, or disease of an individual or group of people. Tweets that violate this policy contain violent threats or glorify violence. Glorifying violence We consider glorification of violence to be behavior that condones or celebrates violence (and/or its perpetrators) in a manner that may promote imitation of the act. We also prohibit the glorification of violence where protected categories have been the primary target or victim." Further, Twitter's advertising policy states: "Twitter prohibits the promotion of weapons and weapon accessories globally. Examples of weapons and weapon accessories include: Guns, including airsoft guns, air guns, blow guns, paintball guns, antique guns, replica guns, and imitation guns Gun parts and accessories, including gun mounts, grips, magazines, and ammunition Rental of guns (other than from shooting ranges)." Based on your own terms of service, we urge you to proactively prevent the online dissemination of these dangerous 3-D designs across all of your platforms. Doing so will make all of our communities safer. Please also let us know if there are ways that we might be able to assist in this endeavor.

First published on Aug. 17, 8:17 a.m. PT.

Updates, 11:49 a.m. PT: Adds Reddit spokesperson statement, Twitter's response and background information regarding the matter.

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