The Murder at Midnight

Keith Narford glanced up at the clock in his office and laughed nervously. Eleven fifty-five. Then he looked down at the desk and at the note that lay there. Printed on the note in neat capitals were the words, 'YOU WILL DIE AT MIDNIGHT'.

He had recieved the note in the mail that morning, but it had all begun a week ago. Keith Narford, people-smuggler, had been in his office when the telephone rang. He picked it up.
"Keith Narford, because of you ten people died in the back of a lorry a week ago. You will pay." The voice suprised him, it was the voice of a young girl.
"What is this? I won't confess to anything..."
"You won't need to. I know. And that is why you will die in a week's time, at midnight."
Then the 'phone went dead. Keith Narford stared at the 'phone, then he shrugged it off. Probably some cop trying to scare him into a confession. Well, Keith Narford didn't scare. He put down the telephone and went on with his work, work relating to his ostensible respectable road haulage business.
By the following morning he had forgotten all about the threatening message. When the telephone rang he did not expect the same voice to come through the wires:
"You will join the many you have killed at midnight in six days."
"Who are you?" He remembered. "What do you want?"
"To kill you, Keith Narford."
"What the...?"
"Yes, Keith Narford. You have killed many people. Remember that truck from Belgium? The one with the faulty refrigeration unit? The one that poisoned twelve people, two men, three women and seven children? They paid you all the money they had for a better life in this country. Instead you killed them. Killed them by putting them inside that truck. Don't deny it, I know. And I'm not the police."
The line went dead again. Narford reeled in horror. A death threat! Someone was going to kill him!
Think, he said to himself. You've got nearly a week to prepare, to get away.
No. He decided he could not flee the country. The police were already suspicious of his activities, and flight would confirm their suspicions. Anyway, the killer might follow him. So what COULD he do?
Make the office secure. Alarms, armoured doors, bullet-proof blass. Every possible entrance to the room would be made secure. He'd be safe!

And so, a week after that first 'phone call, Keith Narford watched the clock tick towards midnight. He looked at the armoured door, the bulletproof glass, the evidence on the ceiling of where that had been armoured. There was no way in.
"I've done it!" he cried as the minute hand of the clock ticked towards midnight.
"No."
Keith Narford stared in horror as something materialised in his office. It was under five feet tall, black and smoky, and the only discernable features on its face were two glowing red eyes.
"No!" Narford screamed, jumping to his feet, pulling a gun from his desk. "What are you?"
"I am the Outsider. I avenge crimes which the police cannot touch. I know your secrets, andI know that you deserve to die, so I shall kill you."
She raised one arm. Narford was rooted to the spot in terror. He heard something, then he looked down. A crossbow bolt stuck out of his chest.
Then Keith Narford fell to the floor, dead.

They found his body in the morning. Sealed inside a bomb-proof room, locked from the inside. It took the police four hours to get inside.

How do I know? Well, the Outsider came in late this morning. She'd been inside the room from the minute Narford entered it, invisible. And she left when the police opened the door.