WaPo:

Most high-profile attempts at voter suppression target minority groups that are traditional Democratic constituencies. That’s why the latest pre-election news from Florida is in the man-bites-dog category. The FBI and state law enforcement are investigating letters – most of them sent to Republicans, including the president of the Jacksonville City Council and a former U.S. ambassador — telling them that they will be unable to vote unless they can prove American citizenship.Chris Cate, spokesman for the Florida Division of Elections, said Wednesday that his agency has received reports from 28 counties (including Broward, Duval, Hillsborough and Palm Beach) about the letters that were mailed with a Seattle postmark dated Oct. 17. He said authorities do not know how many letters were sent.

The letterhead sports the head of a bald eagle silhouetted against an American flag. The name of the appropriate county elections supervisor is also included. The letter says that the county has received information from the state “bringing into question your eligibility as a registered voter.” It directs the recipient to “please stop by our main office with any original documentation that demonstrates U.S. citizenship.” In bold lettering, it adds: “A nonregistered voter who casts a vote in the state of Florida may be subject to arrest, imprisonment and/or other criminal sanctions.”

I'm almost sure this is a prank ("What makes the mailing particularly unusual is that it targets some fairly prominent Republicans..."), and, if such is the case, I can assure you The Stranger had nothing to do with it.