Elton Simpson and Nadir Soofi were shot and killed May 3, after they allegedly opened fire first on a crowd gathered for a Prophet Muhammad cartoon contest.
The postmark on the letters indicate they were mailed from Oklahoma City, but the return address was bogus.
The 8-paragraph letter contained insults about the Prophet and went on to threaten violence against the Muslim community. They said, quote: "This is an eye for an eye and a tooth for a tooth warning."
It went on to threaten the destruction of Mosques saying "Their burning to the ground would bring great recompense, especially if they were full of Muslims."
The letter was signed "Most sincerely, and severely yours, patriot friends all over America."
The President of the Islamic Center in Phoenix says they have received several threats in the past, but the language of this particular letter concerned them. But the President says he was not personally offended by its disparaging remarks.
"Everybody has the right to criticize and say whatever they want, everybody has the right to be a bigot, to be an idiot, so this letter indicates is this person is that type of person, he's a bigot, he's a racist, and he's an idiot," said Usama Shami.
Phoenix Police have stepped up patrols around the Mosque, especially on Fridays which is a day of worship for the congregation.
On Fridays, there is a Phoenix Police Officer parked in front of the Mosque.
The FBI says they are working to find the source and determine the credibility of the threat.