Local officials in Colorado acknowledged “very serious” voter fraud after learning of votes cast in multiple elections under the named of recently-deceased residents.

A local media outlet uncovered the fraud by comparing voting history databases in the state with federal government death records. “Somebody was able to cast a vote that was not theirs to cast,” El Paso County Clerk and Recorder Chuck Broerman told CBS4 while discussing what he called a “very serious” pattern of people mailing in ballots on behalf of the dead.

It’s not clear how many fraudulent ballots have been submitted in recent years. CBS4 reported that it “found multiple cases” of dead people voting around the state, revelations that have provoked state criminal investigations.

“We do believe there were several instances of potential vote fraud that occurred,” said Colorado Secretary of State Wayne Williams. “It shows there is the potential for fraud.”