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.