(AP) – Police are investigating a threat against a number of public officials in Wisconsin, including Gov. Scott Walker and Lt. Gov. Rebecca Kleefisch, a person within the Republican party told The Associated Press on Wednesday.

The source was briefed on the threat but requested anonymity because the person was not authorized to release the information.

Word of the threat came when state Senate Chief Clerk Robert Marchant sent a memo to all Senate offices warning senators to be vigilant about their safety. The memo said the threat was against various Wisconsin public officials, including two state senators, but did not name them.

The memo said the FBI, the Capitol Police and local law enforcement officials were investigating. Marchant did not elaborate on the nature of the threat, though he told Senate staffers to monitor online postings that mention them or their senators, suggesting the threat was Internet-based.

The governor’s office referred calls to the Capitol Police. Kleefisch did not immediately return messages.

Officials with the state Justice Department, Capitol Police and the FBI all declined comment, saying the matter was an ongoing investigation. Madison Police Lt. Mary Ricksecker said she had not heard about any threats.

Marchant declined comment.

Andrew Welhouse, a spokesman for Senate Majority Leader Scott Fitzgerald, R-Juneau, declined comment. John Jagler, a spokesman for Assembly Speaker Jeff Fitzgerald, R-Horicon, said he hadn’t heard anything about the threat and didn’t believe it extended to any Assembly members. Jeff and Scott Fitzgerald are brothers.

News of the threat comes four days after an assassination attempt on U.S. Rep. Gabrielle Giffords in Arizona.

Prosecutors say 22-year-old Jared Lee Loughner shot Giffords, killed six other people and wounded 13 more as Giffords addressed constituents in front of a Tucson grocery store. Giffords is expected to survive.

The incident has sparked debate over divisive political rhetoric.