The hack of the Florida contractor comes on the heels of hacks in Illinois, in which personal data of tens of thousands of voters may have been stolen, and one in Arizona, in which investigators now believe the data of voters was likely exposed.
The FBI, in the coming days, is preparing to provide updated guidance to state elections officials around the US aiming to help them spot suspicious activity on their computer networks. Several states have reported attempted scans of their computer systems, which often is a precursor to a breach.
Previously, Illinois officials have said data on fewer than 90,000 people may have been affected by a breach there, and Arizona officials said they saw no indications hackers accessed data in their systems.
The vendor hack in Florida prompted the FBI last week to coordinate an emergency call with county election supervisors who operate the election system in the perennial battleground state. CNN has not confirmed the name of the vendor that suffered the attack.
ABC News first reported that Florida election information was compromised.
"Investigators believe a local contractor in California was the target of the hackers, but the systems accessed weren't related to the elections," US officials said.
A spokeswoman for the Florida Secretary of State said: "We currently have no indication of a Florida-specific issue. The Florida Voter Registration System database is secure. The Department of State does not utilize a vendor for voter registration services. The Department has in place many safeguards to prevent any possible attempts from being successful."
An FBI spokeswoman said the bureau and the Department of Homeland Security hosted a conference call with Florida state officials to address questions regarding the security of election systems and to share information regarding the general nature of the cyber threat. FBI and DHS continue to work closely with state officials to assist them in safeguarding their election infrastructures.
In the case of Arizona, US officials say the working assumption by investigators is that hackers were able to access data, even if there are no signs of tampering. Arizona officials maintain they've found no signs that hackers got in.
"We have no updates, our story hasn't changed," a spokesman for the Arizona secretary of state said. "We have seen no access into statewide registration database and no manipulation of that database."
FBI investigators believe the the hacks and attempted intrusions of state election sites were carried out by hackers working for Russian intelligence.
The cyberattacks on election registration sites are focused on parts of the US election system that wouldn't affect the votes cast or the vote counts, according to US officials. Instead, the intruders are targeting registration systems.
In a statement last Friday, the Director of National Intelligence and the Homeland Security Department formally blame Russia for hacking political organizations, including the Democratic National Committee, and orchestrating the release of private emails in an attempt to meddle in the US elections.
The statement said the US government wasn't yet ready to attribute the hacks of election registration sites.