JACKSONVILLE, Fla. -

Twenty Florida counties now report letters mailed to voters claiming to be from the supervisor of elections telling people they may not be eligible to vote.

The letters, first reported in Duval, St. Johns and Brevard counties on Monday, have the supervisor of elections letterhead and tell the voter he or she may not be a U.S. citizen and they must return a form within 15 days or their name will be removed from the voter roll. The form requests personal information, including the voters' Social Security number.

The letters, postmarked in Seattle, Wash., are not from the supervisor of elections, have no signature and are not real.

Duval County Supervisor of Elections Jerry Holland sent out an alert "warning voters to be aware of a fraudulent letter instructing voters to prove they are a citizen of the United States." Vicky Oakes, St. Johns County Supervisor of Elections, issued a similar statement Monday afternoon.

On Tuesday, Florida Secretary of State Ken Detzner said there are reports of similar letters being received by voters in Brevard, Broward, Collier, Clay, Escambia, Hillsborough, Indian River, Jackson, Lee, Liberty, Orange, Osceola, Palm Beach, Pinellas, St. Lucie, Seminole, Sumpter and Volusia counties.

The Florida Department of Law Enforcement and the FBI is now investigating the source of the letters.

"If you get it also and it has a return address from Seattle, Wash., and it's saying it's from the elections office, don't even open it. Bring it to us," Holland said. "The officials would like to fingerprint the letter."

Holland said he has reports of 12 letters being mailed to Jacksonville voters and state elections officials report about 50 documented cases. At first it appeared that only Republican voters were targeted, but the secretary of state says registered Democrats have also received the letters.

Because of recent stories in the news that the state of Florida is trying to purge voters who are not U.S. citizens, the letters may seem more valid.

"Anytime anybody's trying to prevent somebody from voting, that's a serious issue," Gov. Rick Scott said Tuesday. "In our state we have zero tolerance.  We want everybody to vote."

The Florida Division of Elections urges anyone receiving one of the letters to contact its voter fraud hotline at 877-868-3737 or visit Election.dos.state.fl.us.