In a letter sent to Twitter, Senators Dianne Feinstein (D-Calif.), Edward J. Markey (D-Mass.), Bill Nelson (D-Fla.), Robert Menendez (D-N.J.) and Richard Blumenthal (D-Conn.) are urging the platform to take steps to proactively block or limit people's access to 3D-printed gun designs. They also sent similar letters to Facebook, Microsoft, Craigslist, Google, Reddit and Yahoo.

The Senators called reports that thousands of CAD files had already been downloaded and can now be freely distributed "deeply troubling." They said those could put "untraceable and undetectable" deadly weapons in the hands of any person, thereby undermining firearms laws and putting communities at risk.

To back up their argument, the group cited sections from Twitter's terms of service and rules that allow the company to delete content like 3D-printed gun designs from its website. They pointed out that Twitter says it reserves the right to remove content that violates user agreement, including unlawful conduct, and that it prohibits people from using its platform to further illegal activities. Tweets that "glorify violence" are also against its rules. Most importantly, Twitter says it "prohibits the promotion of weapons and weapon accessories globally."

After several states sued to block sales of 3D-printed weapon designs online, a Seattle judge granted the temporary restraining order eight state attorney generals and the District of Columbia were seeking. It will only be in effect until the end of August, though, and as the Senators noted, CAD files are already in circulation. It might be up to the tech giants to make sure they're not so easy to find.