Colorado Secretary of State Scott Gessler says a list of nearly 4,000 registered voters he suspects are not citizens is not subject to public inspection because it is part of an investigation.

Gessler on Tuesday denied an open records request from The Associated Press to review the list. The secretary mailed letters to the voters last week asking them to verify their citizenship or voluntarily withdraw from the rolls.

Gessler's office said the list is a record of an ongoing administrative investigation.

Voter rights group Common Cause has criticized Gessler's initiative, saying it cannot ensure that eligible voters will not be removed from the rolls.

Gessler spokesman Rich Coolidge says the goal is improve the integrity of the voting rolls and not draw attention to individuals.