Sini Mathews, the mother of a 3-year-old girl found dead in a Richardson culvert in October 2017, is free after spending 15 months behind bars awaiting trial on child endangerment charges, according to the Dallas County District Attorney's Office.

The charges against Mathews, who faced between two and 20 years in prison, were dropped Friday by prosecutors. Her trial was expected to begin next month.

In a letter to the 282nd Criminal District Court, the Dallas County DA wrote that "after extensive investigation it has been determined that the state cannot prove this matter beyond a reasonable doubt at this time ... the state respectfully requests that this case be dismissed without prejudice."

Mathews had been charged with child endangerment after prosecutors said she left her daughter home alone while she and her husband Wesley went to dinner with their 4-year-old biological daughter on the night before the child was reported missing

Two weeks later the child’s body was found in a drain pipe near the Mathews family home.

Sini's husband, Wesley Mathews, still faces a capital murder charge in connection with his daughter's death and he's being held on a $1 million bond.  His trial is set for May.

Sini's attorney, Heath Harris, said he plans to fight to get his client's passport returned and to restore visitation with her biological daughter who is living with family in Houston. Eventually, Harris said, they'll work to restore Sini's parental rights.