"We're here today with a lot of humility and some sadness and even shock," interim Senate Majority Leader Geoff Michel said Friday.
Michel said he and other senators heard from a number of staff members over the past two weeks that Koch had been involved with one of her direct subordinates, but were stunned at her sudden resignation, the Minneapolis Star Tribune reported.
"Her response to the conversation was ... 'I think I need to consider resigning,'" said Assistant Senate Majority Leader David Hann, R-Eden Prairie.
Michel said after learning of the allegation, Senate leaders made a quiet investigation into the matter.
"There is no doubt that a manager cannot have such a relationship with someone they oversee, whose budget they oversee," Michel said. "It's pretty clear. That kind of relationship is inappropriate, it raises a conflict of interest and it creates ... an unstable, unsustainable work environment for our staff.
"I think in the end there are probably only two people who really know what kind of relationship [it was] and how long it had been happening," Michel said. "It certainly had risen to a level within our Senate family that people were coming to us."
On Thursday, Koch said she planned to serve out her term.
"I would say she's probably got some more thinking to do about her future," Michel said, adding he didn't know if ethics charges would be made.