Sen. Bill Nelson's claim that Russians have "penetrated" some Florida voter registration systems drew increasing backlash Thursday with the state's chief elections officer demanding information from officials in Washington and others saying they have seen no evidence supporting the assertion."Let me be clear, this is a very serious charge made in a public setting without any evidence, details or any prior communication to state or local election officials in Florida," stated a letter to the Senate's Intelligence Committee by Secretary of State Ken Detzner, who was appointed by Nelson's opponent in this year's senate race — Gov. Rick Scott.In response to Detzner's letter, Nelson stood by his earlier claim."For months, (Sen. Marco Rubio) and I have been warning Florida's elections officials about the threat they face and urging them to reach out to the federal government for help," Nelson said in a statement. "I hope the appropriate federal officials find a way to immediately provide them all the information they can to protect our elections."The back and forth illustrates heightened sensitivity to security issues following the 2016 election and communication gaps that appear to exist between intelligence gatherers and stakeholders. Information sharing is hindered by the lack of security clearances for county officials to obtain classified briefings, causing confusion when unsubstantiated claims are made.Nelson's comments, coming before a campaign event in Tampa on Wednesday, drew national attention. He said Russians had "penetrated certain counties in the state," emphasized he was talking about the current election cycle, and attributed his knowledge to the Senate Intelligence Committee. He will not say how many counties, citing classified information."We were requested by the chairman and vice chairman of the Intelligence Committee to let the supervisors of election in Florida know that the Russians are in their records," Nelson told the Tampa Bay Times, referring to a July letter he and fellow Florida Sen. Marco Rubio sent to Florida's 67 county election supervisors. The letter is strongly worded but does not describe known hacking.Nelson said he was raising alarms so that county officials could take steps to shore up their security ahead of the midterm elections. He is on the November ballot, facing the toughest challenge of his four-decade career from Scott, whose administration said it had "no evidence" to corroborate the claims. Detzner amplified that response in his letter to the Republican chairman of the Senate Intelligence Committee, Sen. Richard Burr of North Carolina, on Thursday evening, seeking a response by Monday, the start of early voting.Detzner faced intense criticism from county election supervisors for not applying for federal election security money until late May. It took the personal intervention of Scott to force the state to seek its $19 million share, despite repeated warnings from federal authorities that Florida was a likely target of more meddling.A U.S. Department of Homeland Security spokeswoman Sara Sendek also said "we have not seen any new compromises by Russian actors of election infrastructure," while adding, that officials don't need to "wait for a specific threat to be ready" and that they and Florida's election officials have taken steps to secure existing systems.Some elections officials remain incredulous about Nelson's claims. A number of large counties, including Pinellas, Pasco, Seminole, Broward and Miami-Dade, have issued statements saying they are not aware of any breaches.Yet still, those Senators who have access to classified information on election security neither confirmed nor denied what Nelson said. Burr declined comment Wednesday. Vice Chairman Mark Warner, D-Va., issued a statement that did not directly address Nelson's assertions."Russian activities continue to pose a threat," Warner said. "I hope all state and local elections officials, including Florida's, will take this issue seriously."Rubio, a member of the Intelligence Committee, has not responded to requests for comment and has generally been careful with his words, saying he cannot discuss classified information.Rubio has spoken privately with some county election officials while publicly warning about overconfidence, calling Florida a "beacon" for interference attempts. The state was targeted in 2016, according to federal authorities, though there is no public disclosure of a breach. Rubio describes that effort as "probing" system vulnerabilities for future elections.Nelson and Rubio have been planning to jointly appear on the Senate floor to discuss the issues and both described a hypothetical in which hackers could alter voter registrations in some counties.Rubio has emphasized the need for more information sharing and pushed for language in the intelligence authorization bill to facilitate security clearances for county officials, some of whom have complained that Congress hasn't done enough on that front."This is like me telling you, 'Watch out. Watch out for what? Well, I can't tell you.' How does that help anybody?" said Okaloosa County Supervisor of Elections Paul Lux.Nelson's comments caused anxiety among some Florida voters."Our call center was literally inundated all morning with calls from people concerned about what they heard from Sen. Nelson," said Pasco County Supervisor of Elections Brian Corley. "We're in a position of having to quell voters' concerns."