Eric Frank Russell's "Venturer of the Martian Mimics" aka "I, Spy" aka "Spiro" (novelette, detective, free): Exploits of a Martian spy on earth

This is a detective story: Chief Inspector McKechnie of Scotland Yard, with help from Dr Lanigan, solving the case of very unusual phenomena happening around London & then in rest of Europe: a man murdered by electrocution several miles from nearest power lines, then he's seen boarding a bus & a train several hours after his death! A shape shifter that gives electric shocks to anyone touching it. Someone breaking into sensitive installations clearly for espionage purposes. They'll eventually kill the uncatchable.

There is no mystery for reader, though. We see the alien land & shapeshift in the first scene itself. Still a good read; I finished it in a single sitting.

Fact sheet.

First published: Tales of Wonder, #12 (Autumn 1940).
Download full text as part of scans of the Weird Tales, May 1947 (Canadian Edition) where it was reprinted.
Rating: A.
Related: Stories of Eric Frank Russell (annotated & ranked list).