Harry Harrison: Arm of the Law

It was a big, coffin-shaped plywood box that looked like it weighed a ton. This brawny type just dumped it through the door of the police station and started away. I looked up from the blotter and shouted at the trucker's vanishing back.
"What the hell is that?"
"How should I know?" he said as he swung up into the cab. "I just deliver, I don't X-ray 'em. It came on the morning rocket from earth is all I know." He gunned the truck more than he had to and threw up a billowing cloud of red dust.
In the box the letter accompanying it said " ... robots, correctly used will tend to prove invaluable in police work ... they want us to co-operate in a field test ... robot enclosed is the latest experimental model; valued at 120,000 credits." So begins the saga of Ned, experimental robot policeman.