The Office of Defense Trade Controls Compliance demanded that the design be removed, and a probe to establish whether the company broke arms control laws relating to overseas shipping is underway.
The design is still being hosting by other sites however, such as the Pirate Bay. Mega's controversial founder, Kim Dotcom, opted to order his staff to remove public copies of the designs from the file locker's servers after being made aware of them.
He denied that he had been contacted by anyone from the U.S. government about the matter, and said the decision was made out of public concern.
"I think it's a serious threat to security of the community. I think it's scary that people can print 3D guns that can't even be detected by metal detectors ... This should concern everybody," he told Radio New Zealand.
In reality, the 3D gun design would still require a fairly expensive printer to fabricate, so the feasibility of a 3D printing enthusiast making a working gun is unclear. The technology is advancing at a rapid pace though, so you can expect legislators to move on this question quickly.