The female athlete says she was sexually assaulted by a baseball player.
The lawsuit claims several former classmates of that baseball player warned university leaders about sexual abuse allegations against him in the past.
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The lawsuit further claims that the lacrosse player reported the assault, but no action was taken.
Northwestern says it first received the allegations in January, and immediately reported them to the Office of Civil Rights and Title 9 Compliance.
The school says the allegations are under investigation.