(OVERLAND, Mo., AP) — The mother of a 6-year-old St. Louis County girl is demanding answers after the child was placed on the wrong school bus and dropped off alone.

Ritenour School District officials tell KMOV-TV (http://bit.ly/2b27lMm ) that a series of mistakes are to blame.

Marvin Elementary School first-grader Wednesday Nesbit was placed on the wrong bus Monday afternoon. The driver dropped her off at the wrong stop and left her alone, violating district policy for a student in kindergarten or first grade.

Amy Nesbit says her child was missing for 90 minutes before two women saw her and contacted police, who picked up Wednesday.

An official with First Student, the school bus operator, called the incident unacceptable and says an internal investigation is underway. Ritenour is also investigating what happened.

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Information from: KMOV-TV, http://www.kmov.com