The letters, critical of DFL Senate candidate Al Franken, appeared in the Minnesota Daily and the Winona Daily News.
Coleman campaign spokesman Cullen Sheehan said one of his staffers sent the letters to at least two people who then sent them to the newspapers under their own name.
Sheehan said the letters should not have been sent to the newspapers the way they were.
"It's a mistake, but in the realm of the campaign, it's a mistake you learn from, and you move on and it's not going to happen again."
The Franken campaign said it wants to know if the Coleman campaign has used similar tactics in the past.