Drained and spectral, everything in the work of Luc Tuymans seems barely able to survive. The very first room of his retrospective at Tate Modern makes you shudder, like someone blundering on to the scene of a callous and inexplicable crime. These paintings, isolated by clinical expanses of white wall, belong to a series called Illegitimate. Hints of flowers can be detected in one image, struggling to emerge from the acid green brushed so drily on to the canvas. But the colour appears toxic, and threatens the viability of the female reproductive organs suggested in another, even more tantalising picture. And in Illegitimate III, the lithe body of a leotard-clad performer is mocked by the absence of a head. What is going on?
In the next room, we see a young girl's leg and a slice of grey skirt above it. But no more than that. There is a prevailing sense of tension and disquiet. Sometimes the titles of the works take us further and startle with their apparent frankness. Child Abuse alerts us to the ominous implications of one arid painting, which is invaded by the form of a pale-blue tulip. Yet the old, faded photographs it contains look almost indecipherable. Likewise, in a painting called Bloodstains, we are left to speculate on why dark red orbs spatter the surface.
Before long, the show confronts us with images that derive from a particular, not to say traumatic, source. At first sight, the big canvas of a skier sitting in the snow seems placid enough. Much of the picture space is filled with purged whiteness, and even the man at the centre is reduced to stark, monochromatic simplicity. Then we notice that his face is nothing more than a blank, as if the act of falling has robbed the hapless skier of his iden-tity. But Tuymans tells us exactly who the tumbled figure is: Hitler's favourite architect, Albert Speer, whose image here comes from his wife's film of their skiing holiday. Suddenly, the painting's apparent purity turns sour.
Although he was born more than a decade after the end of the Second World War, Tuymans is haunted by the Holocaust. But he insists on approaching it obliquely. Rather than depicting the bestiality head-on, he presents us with a painting called Walking, in which a group of diminutive fascists make their seemingly harmless way through picturesque snow near Hitler's Alpine villa. By viewing them from afar, Tuymans makes us appreciate how the Nazis cloaked their barbarism in a veneer of civility.
He approaches the terror of the concentration camps with equal stealth, depicting objects as humdrum as a tooth, a lampshade and a laboratory window with quiet, almost hesitant brush strokes. In reality, these images are all bound up with disgusting operations, carried out by the Nazis on prisoners' bodies under the spurious guise of scientific inquiry.
Only in a few images from his extensive Holocaust series does Tuymans become more openly agitated. In one distressing work, slender hands wave and bend in front of dead-looking eyes. Arranged in a grid formation with cruel, cell-like black bars dividing them, these poignant images are based on photographs by a doctor who coldly conducted experiments on gypsy twins.
In a chilling canvas identified with blunt directness as Gas Chamber, Tuymans finally enters the epicentre of the Holocaust. Nothing remains of the humans who were exterminated in this forlorn interior. But the drain set into the floor is sickeningly suggestive, as are the dark cavities punctuating the ceiling. The room's fundamental repulsiveness is conveyed above all by the smeared brush marks that Tuymans applies.
On the whole, though, Tuymans keeps himself at a distance from the heartland of horror. Fascinated by the endless human capacity for deception, he focuses time and again on attempts to disguise the true extent of Nazi infamy. Our New Quarters refers to three yellow words scrawled, in English, on a fake postcard of a camp created in Czechoslovakia to fool visi-tors. They were allowed to inspect the premises, deluded into imagining that the "model" buildings and carefully staged recreational activities were typical of concentration camps elsewhere in Europe. Painted for the most part in a deep green, which seems to envelop everything in submarine gloom, Our New Quarters is a profoundly depressing image. It warns us never to take anything at face value, and the rest of the exhibition alerts us, at every turn, to secrets, obfuscation and a corrosive refusal to admit the truth.
This is a deeply disconsolate body of work, bearing with relentless scepticism on the failings of our time. Tuymans makes no attempt to ameliorate, let alone disguise, his pessimism. Dogged by a continual sense of fear, he lets it invade everything produced in his Antwerp studio. As a result, he does not need to preach or underline his misgivings in an obvious way. After the nightmare of the 9/11 terrorist attacks, Tuymans was widely expected to paint a major meditation on the atrocity. He certainly displayed a monumental new canvas at the Kassel Documenta a few months later, but to everyone's surprise it contained a radiant still life. Far from offering an affirmation, though, both jug and fruit look unaccountably frozen, marooned in a pale expanse of emptiness on every side. They hover uneasily in space, ready to fade from sight or collapse with unnerving speed.
Tuymans is obsessed by a shameful lack of moral substance in contemporary life. The overall feeling is summed up by a small canvas called Window. It shows only a scattering of pale grey smears. Like the aftermath of a violent rainstorm, they have been left to dry, by degrees, on the murky and unrevealing glass.
"Luc Tuymans" is at Tate Modern, London SE1 (020 7887 8888) until 26 September. Earlier Tuymans shows are discussed by Richard Cork in his recent four volumes of writings on art (1970-2000) published by Yale