Colorado Republican Secretary of State Scott Gessler is alerting prosecutors of 155 suspected noncitizens who have cast ballots.
The announcement Monday continues a contentious debate between the state's election chief and voting rights groups and Democrats who question the validity of his findings.
Gessler's office says the voters are among 4,201 people who received letters asking them to clarify their status since last summer. Gessler says the 155 have voted in one or more past elections, and did not reply to the letters.
Letters were sent to people who once showed proof of non-citizenship, such as a green card, when getting a driver's license and then later appeared on voter rolls. Gessler is also using a federal immigration database, but critics say it's not error-proof.