I don't think it's right wing Americans. Planned Parenthood would be a very likely target of Muslim fundamentalists who have been upset for years at the genocide being done to them by International Planned Parenthood using UN money. Clinton did a great deal of damage by helping the feminists take over many of the US policies and in the UN. They were forcing abortion clinics onto countries that didn't want them, along with radical feminism and the homosexual agenda.
Radical liberals do not want to realize it, but it was not Conservative Judeo-Christian American values that got us into this mess. But those values will have to get us out.
It would not surprise me one bit if Muslims would mail these bioterrorism letters with messages imitating ultra-right crypto-Christian rhetoric as a tactic to try to divide America.