'She wasn't THERE' Agatha Christie 'A murder is announced'

Illustration for detective book, which I find incredible!

About the novel: one day in the old countryside there was a strange announcement on the newspapers that was called 'A MURDER IS ANNOUNCED!'. It said to come at the 6:30 p.m. to the Letitia Blacklock's house, where would be this 'murder'. The guests didn't believe that, but even so they gathered in the old lady's house. When the time was 6:30 p.m., suddenly, the lights turned off, and unknown man broke into the living room with the torch and a gun. Then, 2 shots fired, and after that guests were frightened! Then came 3rd shot and man fell down to the floor. Afterwards, when the lights were turned on, people discovered that the man was dead, and 2 shots were fired to the old lady Blacklock, who, fortunately, wasn't injured. But who and why did that? Missis Marple and mister Craddock will discover that in this story...