The wrong letter

The telephone bell rang on the table of Superintendent Sinclair at Scotland Yard. He was a busy man, and had given orders that he was not to be disturbed except on matters important.
Putting down a paper he had been reading, he picked up the receiver. A woman’s voice spoke.
“Is that Scotland Yard?”
“Yes, yes,” he said impatiently, “Superintendent Sinclair speaking, what is it?”
“Listen carefully,” said the voice. “The Home Secretary has been murdered at his own house, it would be as well if you...