Washington (CNN) The House Committee on Ethics announced Wednesday it is opening an investigation into allegations that Democratic Rep. Katie Hill of California engaged in an improper relationship with a congressional staffer in possible violation of House rules.

In a separate statement to constituents released on Wednesday, Hill admitted to having an inappropriate relationship with a campaign staffer before entering Congress, for which she apologized.

Hill on Tuesday denied having a relationship with Graham Kelly, who currently works as her legislative director, according to Legistorm, a private research organization dedicated to tracking Congress. There is no evidence of that alleged relationship.

"Allegations that I have been involved in a relationship with Mr. Kelly are absolutely false," Hill said in a statement.

CNN has reached out to Kelly for comment, but has not heard back.

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