It was the State Department. The alleged offense, a possible export law violation:

On Thursday, Defense Distributed founder Cody Wilson received a letter from the State Department Office of Defense Trade Controls Compliance demanding that he take down the online blueprints for the 3D-printable “Liberator” handgun that his group released Monday, along with nine other 3D-printable firearms components hosted on the group’s website Defcad.org. The government says it wants to review the files for compliance with arms export control laws known as the International Traffic in Arms Regulations, or ITAR. By uploading the weapons files to the Internet and allowing them to be downloaded abroad, the letter implies Wilson’s high-tech gun group may have violated those export controls.
“Until the Department provides Defense Distributed with final [commodity jurisdiction] determinations, Defense Distributed should treat the above technical data as ITAR-controlled,” reads the letter, referring to a list of ten CAD files hosted on Defcad that include the 3D-printable gun, silencers, sights and other pieces. “This means that all data should be removed from public acces immediately. Defense Distributed should review the remainder of the data made public on its website to determine whether any other data may be similarly controlled and proceed according to ITAR requirements.”

Wilson will remove the files, he said, until State has performed its reviews, though somehow I doubt State will be quick or straightforward about the determination process, lest it allow Wilson to put the plans back up. But this is the Internet, man. The plans for the “Liberator” handgun were downloaded 100,000 times in two days.
Laying aside how anyone feels about the printable gun in particular, it is a particularly illustrative example of how fast technology can outstrip regulation, leaving bureaucrats in the dust as free people invent things regulators had never even contemplated. As technology gets better faster, it’s a train running far ahead of regulators’ ability to lay down track. To some, this is petrifying. To others, exciting. The reality of a 3-D printable gun has led to a bunch of predictable pants-wetting over what to do about it. But what’s more interesting, and relevant to anyone who’s keen on the huge societal development that is home manufacturing, are the ways liberals are already dreaming up to thwart the 3-D gun.
The battle over these plans touches on the Second Amendment, of course, but the distribution of 3-D printing plans is a First Amendment issue, which will make regulators sad about their impotence. Reasonmagazine lays out some of the statist fantasies already being indulged as possible remedy. They turn, ironically, to the hope the gun industry will back legislation to crush 3-D guns as record companies did to crush file sharing. Or, failing that, maybe censorship and burdensome regulation of in-home printers?

Manjoo then appeals to the “3-D gun movement’s fundamental error—their belief that information can’t be controlled.” That’s right, he sees salvation through the power of censorship:

[I]f the authorities set their mind to it, they can bankrupt you for sharing songs online. Countries where guns are already strictly curbed could impose similarly harsh measures against the distribution of plans for 3-D guns—and if they enforce them strictly, they might well limit their availability.
Frankly, I have a hard time believing that U.S. courts that found the sharing of source code for once strictly regulated encryption software to be protected by the First Amendment won’t see similar free speech concerns in the sharing of firearms designs on the Internet. And the Internet is a world-wide network; you need only one jurisdiction friendly to free speech to defeat censorship efforts elsewhere. Internet censorhip has never been successful, and it seems a faint hope for controlling the distribution of 3D printer designs.

Manjoo then suggests curbs on 3D printers:

[I]t’s conceivable that lawmakers would impose severe restrictions on the 3-D printer industry, which, of course, isn’t protected by the Second Amendment. Lawmakers could require 3-D printer manufacturers to prevent their machines from printing certain files—in the same way your DVD player can’t play movies from a different region—and impose harsh penalties for circumventing those rules. They could even make you register your printer the way you’ve got to register your car.

Is there anybody left who doesn’t know how easy it is to bypass those region restrictions on a DVD player or computer? Even if some variant of “region restriction” software were built into 3D printers — say, the sort comparable to the type that’s supposed to prevent photocopiers and printers from knocking off currency — it would have an even tougher job of recognizing ever-morphing 3D designs than printers do of recognizing relatively static images of $20 bills. And that restrictive software has been so (un)successful that 60 percent of counterfeit bills recovered in recent years were created on ink-jet printers.

Eh, good luck with that. The “Liberator” is already out there, as will be plans for any number of things your neighborhood busy-body or federal bureaucrat might find objectionable. Defense Distributed says “take it up with the State Department.” But taking it up with a search engine is probably easier. I’m reminded, despite the intense irony of invoking a monarchical phrase, of the sentiment: “The printable gun is dead. Long live the printable gun.”