BURLINGTON, Vt. — Jury selection got under way Monday for the murder trial of a Vermont woman accused of stabbing her niece, in a case marked by the unexplained disappearance of the murder weapon.

Thirty-one-year-old Latonia Congress is accused in the 2009 slaying of 16-year-old Shatavia “CeCe” Alford, who police said was killed in a dispute over laundry.

The killing took place at the apartment the two shared in Essex.

Essex police Sgt. Douglas Babcock testified at an earlier court hearing that he took photographs of the knife believed to have been used in the attack but said it disappeared while he helped rescuers remove Alford from the apartment.

Jury selection began Monday in Chittenden Superior Court criminal division in Burlington.

The missing knife, which police first said was thrown at Alford and later said was used to stab her, is expected to be a key focus of testimony at the trial.

The knife was photographed by police at the scene, but it disappeared after that and neither police nor prosecutors know what happened to it.

Congress’ lawyers rejected a plea deal offered by prosecutors in which she would have accepted a sentence of 25 years to life in prison. A first-degree murder charge carries a minimum sentence of 35 years.

Congress, who has pleaded not guilty, says she was insane at the time of the stabbing and isn’t criminally responsible for it.