It was from a US lab.
Senator Rand Paul, please pickup the courtesy phone
It was not naturally occurring anthrax. Scientists quickly identified it as the notorious Ames strain, which researchers at the U.S. Army lab in Fort Detrick had essentially invented. As PBS’ Frontline program put it in 2011: “in October 2001, Northern Arizona University microbiologist Dr. Paul Keim identified that the anthrax used in the attack letters was the Ames strain, a development he described as ‘chilling’ because that particular strain was developed in U.S. government laboratories.” As Dr. Keim recalled in that Frontline interview about his 2001 analysis of the anthrax strain:We were surprised it was the Ames strain. And it was chilling at the same time, because the Ames strain is a laboratory strain that had been developed by the U.S. Army as a vaccine-challenge strain. We knew that it was highly virulent. In fact, that’s why the Army used it, because it represented a more potent challenge to vaccines that were being developed by the U.S. Army. It wasn’t just some random type of anthrax that you find in nature; it was a laboratory strain, and that was very significant to us, because that was the first hint that this might really be a bioterrorism event.