The girl has been charged with murdering her husband Umar Sani days after their marriage in northern Kano state.
Because she did not understand English, homicide investigator Abdullahi Adamu translated her statement from the Hausa language dominant in the region and gave her the document to sign.
She could not write her name, so "she had to use a thumbprint," he told the court during his testimony on the last day of the prosecution's case.
But others in the region, including relatives of the defendant and the deceased, have rejected the notion that the girl was forced into marriage.
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They have said that 14 is a common age to marry in the deeply impoverished region and that she chose Sani from among many suitors.
A motion by defence lawyers to have the case moved to juvenile court was rejected, despite claims by human rights lawyers that she is too young to stand trial for murder in a high court.
While sharia is technically in force in Kano, law enforcement officials have no guidelines concerning how it should be balanced with the secular criminal codes, creating a complex legal hybrid system.
The trial has been adjourned until February 16.