PORTLAND, Ore. (AP) — Authorities on Friday were investigating alleged ballot tampering by an elections worker in one of Oregon’s most populous counties.

The extent of the tampering in Clackamas County, which primarily comprises Portland suburbs, was not immediately clear.

The Willamette Week newspaper reported that the election worker, who was not identified, filled in Republican bubbles on ballots where preferences had been left empty by voters.

A person with knowledge of the investigation said Willamette Week accurately described the incident. The person was not authorized to discuss the matter and spoke to The Associated Press on the condition of anonymity.

Oregon was the first state to conduct elections exclusively by mail, and this the fourth presidential election in which the system has been used. Despite initial fears that the method would lead to fraud, allegations have been rare.