It can be hard from this distance, 42 years after his killing, to remember how important Martin Luther King was. His burial in Atlanta was a state funeral in all but name. America stopped; even the roulette wheels in Las Vegas were still. And the pursuit of his killer was the biggest investigation of a single crime in FBI history.
The FBI had some self-exculpation to do. It had waged a long campaign of surveillance and rumour-mongering against King, and conspiracy theorists were quick to suspect the bureau of having played a part in his assassination. Even if King's triumphs were behind him, he was moving on to perhaps more threatening battles. "Now our struggle is for genuine equality, which means economic equality," he had said. To the likes of the FBI chief J Edgar Hoover, the segregationist presidential candidate George Wallace, and maybe the assassin James Earl Ray, King represented the worst kind of black man: confident and charismatic, oversexed, anti-capitalist and uninterested in personal profit.
In Hampton Sides's account of the events leading up to King's assassination, and the ensuing manhunt, we are presented with a grippingly arranged drama of stalking and pursuit against a backdrop of political instability and social unrest. Ray came from an Illinois family of habitual criminals and race-haters. He was later described by his father, chidingly but admiringly: "Jimmy had too much nerve for his own good. He tried to go too far too fast." We first meet him breaking out of jail in Mississippi, and follow him on the lam in Mexico and LA in chapters that alternate with those describing King's increasing involvement with the garbagemen's strike in Memphis, Tennessee.
King's first march with the Memphis sanitary workers had ended in a riot. He had to go back and succeed: "The Movement lives or dies in Memphis," he said. Meanwhile Ray had been pursuing his own aspirations, attempting to become variously a barman and a porn director and, more successfully, a drug dealer and sex tourist. He went by aliases. He wanted, he said, a face that "no one could describe".
Sides's research into secondary sources and his marshalling of them is most impressive, with the energy and readability of a thriller as killer and quarry move towards each other. He is also good on the urban panics that followed King's shooting. There were riots, burnings and lootings, and Washington, DC was aflame.
And Ray was back on the lam. He dumped his car and typewriter and radio, going off with a few necessities - his .38 revolver, Polaroid camera, cash, clothes and small stack of self-help books. Ray had an appetite for pulp thrillers, but his favourite book was Psycho-Cybernetics, which had been recommended to him by an LA hypnotist whom he had consulted to improve his self-confidence.
Where the book falls down is in the sketchiness of motivation. We are shown how King died; we never learn why. Yes, we understand that Ray was a vile racist. After stays in Canada and Portugal, he was holed up in a hotel room on the Cromwell Road in London, dreaming
of becoming a mercenary in Rhodesia (he was finally apprehended at Heathrow Airport - "Oh God, I feel so trapped," he said). But why did someone who could plan and execute an assassination, who could forge false identities for himself and repeatedly break out of jails, not have a better plan of escape after the killing? He dumped the rifle in the doorway of a Memphis pinball and jukebox joint and drove away in his white Mustang. We never learn if he had help or financing, nor even why he made the decision to stalk and assassinate King.
The book would also have benefited from Sides reining in his promiscuity with adjectives. He doesn't seem to believe that a noun is ever sturdy enough to get the job done on its own: I can just about accept that Brixton Prison is "storied", but I'm still struggling to grasp how the Memphis cotton industry is a "vaguely druidic enterprise".
Hellhound on his Trail has tremendous verve and carries authority, but we are not quite with the principal figures - it's as though we were eavesdropping on history or, rather, stalking it. As if in Ray's room at Bessie Brewer's rooming house, we seem to be watching King, and his killer, through the scope of a hunting rifle.
Hellhound on his Trail
Allen Lane, 480pp, £25
David Flusfeder's latest novel is "A Film by Spencer Ludwig" (Fourth Estate, £11.99)