WASHINGTON — In the months before the 2016 presidential election, Russia’s military intelligence agency penetrated computer systems in at least one Florida county government and planted malware in systems at a manufacturer of election equipment, the special counsel, Robert S. Mueller III, said in his office’s final report on Russian interference in the election.
The report did not cite any evidence that the breaches compromised election results in Florida or elsewhere, and said Mr. Mueller had left further investigation of the incidents to the F.B.I. and the Department of Homeland Security.
The disclosure of the suspected breach added to accounts of Russia’s systematic effort to access voter-registration rolls and other election systems outlined last year by American intelligence officials and in federal indictments.
The special counsel’s report also cited another attack on computers in Illinois, which had already been reported, while the attack in Florida had not previously been disclosed. The penetration of the election equipment manufacturer, identified elsewhere as VR Systems of Tallahassee, Fla., had been known — but not that malware specifically had been planted. The company makes electronic pollbooks and other devices that help officials run elections, but does not make voting machines.
On Thursday, the Florida Department of State, which supervises elections, said that it had “no knowledge or evidence of any successful hacking attempt at the county level during the 2016 elections.” Officials said that upon learning about the information in the Mueller report, the state contacted the F.B.I. to learn more and the bureau declined to provide details of the county government breach mentioned in Mr. Mueller’s report.
The state said that Homeland Security officials told it in 2017 that Russian attacks on its election systems were unsuccessful.
“The Department maintains that the 2016 elections in Florida were not hacked,” the Florida Department of State said.
Experts have traced Russian efforts to find vulnerabilities in election computer systems in dozens of states before the presidential election, and some now assume that every state was targeted at one time or another.
Mr. Mueller’s report said that officers of the Russian military intelligence agency, commonly known as the G.R.U., targeted computers at state election boards, secretaries of state and county governments, as well as computers used by employees at those agencies.
The report cited only three instances in which the officers actually gained access to computer systems. In the instance in Florida, the report stated, Russian officers sent more than 120 “spearphishing” emails to election officials in Florida counties in November 2016. “The F.B.I. believes that this operation enabled the G.R.U. to gain access to the network of at least one Florida county government,” the report stated.
In Illinois, hackers infiltrated a voter registration database at the state’s elections board in June 2016 and, according to a federal indictment last year, downloaded personal details of roughly 500,000 voters before being detected.
Both federal and state officials have previously said that Russian attacks on election systems did not succeed in altering any votes.
In the third instance at the manufacturer of election equipment, the Mueller report states that G.R.U. agents planted malware in a company computer.
In a statement on Thursday, the chief operating officer of VR Systems, Ben Martin, did not directly address the reported malware plant, saying only that details of the Russian spearphishing attempts had long been known. The company has since engaged private and federal experts to assess its security practices, the statement said.
A National Security Agency report leaked in 2017 indicated that a Russian spearphishing attack probably compromised one employee’s computer, but offered few details.