"Twenty minutes after tasting the soup three grandchildren began to feel sick. Unfortunately, two of them, aged three and one, died before the ambulance arrived," the criminal prosecutor said.
The dead children's sister, Ksenya, was diagnosed with "poisoning from an unidentified substance," Dmitry Molchanov said.
The grandmother, Lyubov, told investigators that she had run out of salt, and saw a container with white powder in the cupboard which she thought was salt. She added the substance to the soup.
Preliminary toxicology results indicated the substance was potassium nitrate which has many uses and is known as saltpeter. It has a white bitter taste and is also known as the 'food of death,' as it was used as an oxidizing agent to produce old-fashioned gunpowder.
A criminal case has been opened against the grandmother on involuntary manslaughter charges.