"Due to the open status of the investigation, the content of the note will not be disclosed at this time", police said in a statement Monday.
The "threatening note" was found near the door to Heller's office.
Patricia Spencer on Sunday night confirmed that "entry did occur" at Heller's office, but only said that the investigation was ongoing.
Sen. Dean Heller, R-Nev., during a press conference where he announced he will vote no on the proposed GOP health care bill, June 23, 2017, in Las Vegas.
Heller is also the only GOP senator up for re-election in 2018 in a state that Trump lost, making him one of the most vulnerable Republicans in the next election. Proponents of the bill have ramped up pressure to get his support.
The vote has been delayed as Sen. The announcement came after McCain's office said he would remain in Arizona next week following a procedure to remove a 2-inch (5-cm) blood clot from above his left eye.