Colorado Secretary of State Scott Gessler says 300 more suspected noncitizens have been found on the state's voter rolls.
They were among 3,903 people who received letters questioning their citizenship in August.
Gessler's office previously said another 141 who received letters appeared to be noncitizens, based on a federal immigration database.
Elections officials did additional checks after obtaining more immigration identification numbers to run through the database.
"It’s unacceptable to have ineligible voters casting ballots in our elections,” Gessler, a Republican, said Tuesday. “We want to ensure the most accurate, reliable elections possible."
It's the latest chapter in a contentious national debate over whether noncitizens are illegally registered to vote. Critics say the checks could disenfranchise eligible voters.
Gessler forwarded the names of the suspected noncitizens to county clerks for any potential challenge at the polls.