Mental illness/violent behavior

There is a woman with a mental illness (bipolar/schizo) and she is in a violent abusive relationship.  They just had a baby a week ago and will be bringing home baby in the next day or so. Social worker was investigating mom but has backed off as of yesterday. I don't want to be the reason this child is taken away but I also don't feel right seeing him go home with his parents knowing whether intentional or not he would not be safe. Why would the social worker back off when the mom talks her way out of the investigation? Or are they still investigating but letting baby go home until something can be done?