Every so often, a famous actor or producer will contact Seymour Hersh, wanting to make a movie about his most famous story: his single-handed uncovering, in 1969, of the My Lai massacre, in which an American platoon stormed a village in South Vietnam and, finding only its elderly, women and children, launched into a frenzy of shooting, stabbing and gang-raping. It won him a Pulitzer prize and hastened the end of the Vietnam war. Mostly, they come to see him in his office in downtown Washington, a two-room suite that he has occupied for the past 17 years. Do they like what they see? You bet they do, even if the movie has yet to be made. 'Brad Pitt loved this place,' says Hersh with a wolfish grin. 'It totally fits the cliché of the grungy reporter's den!' When last he renewed the lease, he tells me, he made it a condition of signing that the office would not be redecorated - the idea of moving all his stuff was too much. It's not hard to see why. Slowly, I move my head through 180 degrees, trying not to panic at the sight of so much paper piled so precipitously. Before me are 8,000 legal notepads, or so it seems, each one filled with a Biro Cuneiform of scribbled telephone numbers. By the time I look at Hersh again - the full panorama takes a moment or two - he is silently examining the wall behind his desk, which is grey with grime, and striated as if a billy goat had sharpened its horns on it.

And then there is Hersh himself, a splendid sight. After My Lai, he was hired by the New York Times to chase the tail of the Watergate scandal, a story broken by its rival, the Washington Post. In All the President's Men, Bob Woodward and Carl Bernstein's book about their scoop, they describe him - the competition. He was unlike any reporter they'd ever seen: 'Hersh, horn-rimmed and somewhat pudgy, showed up for dinner in old tennis shoes, a frayed pinstriped shirt that might have been at its best in his college freshman year and rumpled, bleached khakis.' Forty years on, little has changed. Today he is in trainers, chinos and a baggy navy sweatshirt and - thanks to a tennis injury - he is walking like an old guy: chest forward, knees bandy, slight limp in one leg. There is something cherishably chaotic about him. A fuzzy halo of frantic inquiry follows him wherever he goes, like the cloud of dust that hovers above Pig Pen in the Charlie Brown strip. In conversation, away from the restraining hand of his bosses at the New Yorker, the magazine that is now his home, his thoughts pour forth, unmediated and - unless you concentrate very hard - seemingly unconnected. 'Yeah, I shoot my mouth off,' he says, with faux remorse. 'There's a huge difference between writing and thinking.' Not that he has much time for those who put cosy pontification over the graft of reporting: 'I think... My colleagues! I watch 'em on TV, and every sentence begins with the words: "I think." They could write a book called I Think.'

But we must backtrack a little. Before the office, there is the breakfast joint. Hersh and I meet at the Tabard Inn, a Washington hangout so gloomily lit I could do with a torch. He has poached eggs and coffee and 'none of that other stuff, thanks'. (I think he means that he doesn't want potatoes with his eggs). Like everyone in America just now, he is on tenterhooks. A Democrat who truly despises the Bush regime, he is reluctant to make predictions about exactly what is going to happen in the forthcoming election on the grounds that he might 'jinx it'. The unknown quantity of voter racism apart, however, he is hopeful that Obama will pull it off, and if he does, for Hersh this will be a starting gun. 'You cannot believe how many people have told me to call them on 20 January [the date of the next president's inauguration],' he says, with relish. '[They say:] "You wanna know about abuses and violations? Call me then." So that is what I'll do, so long as nothing awful happens before the inauguration.' He plans to write a book about the neocons and, though it won't change anything - 'They've got away with it, categorically; anyone who talks about prosecuting Bush and Cheney [for war crimes] is kidding themselves' - it will reveal how the White House 'set out to sabotage the system... It wasn't that they found ways to manipulate Congressional oversight; they had conversations about ending the right of Congress to intervene.'

In one way, it's amazing Hersh has anything left to say about Bush, Cheney and their antics. Then again, with him, this pushing of a story on and on is standard practice. Though it was Woodward and Bernstein who uncovered the significance of the burglary at the Watergate building, Hersh followed up their scoop by becoming one of Nixon's harshest critics and by breaking stories about how the government had supported Pinochet's 1973 coup in Chile, secretly bombed Cambodia and used the CIA to spy on its domestic enemies. His 1983 book about Nixon, The Price of Power, is definitive. So far as the War on Terror goes, Hersh has already delivered his alternative history - Chain of Command, a book based on the series of stories he wrote for the New Yorker in the aftermath of 9/11 and following Bush's invasion of Iraq. Among other things, Hersh told us of the bungled efforts to catch Osama bin Laden in Afghanistan; of the dubious business dealings of the superhawk Richard Perle - a report that led to Perle's resignation as chairman of the Pentagon's Defense Policy Board (Hersh alleged that Perle improperly mixed his business affairs with his influence over US foreign policy when he met the Saudi arms dealer Adnan Khashoggi in 2003. Perle described Hersh as 'the closest thing American journalism has to a terrorist' and threatened to sue before falling oddly silent); and of how Saddam's famous efforts to buy uranium in Africa, as quoted by President Bush in his 2003 State of the Union speech, were a fiction. Most electrifying of all, however, was his triple salvo on the abuse of Iraqi prisoners in Abu Ghraib. It was Hersh who first revealed the full extent of this torture, for which he traced the ultimate responsibility carefully back to the upper reaches of the administration. 'In each successive report,' writes David Remnick, the editor of the New Yorker, in his introduction to Chain of Command, 'it became clear that Abu Ghraib was not an "isolated incident" but, rather, a concerted attempt by the government and the military leadership to circumvent the Geneva Conventions in order to extract intelligence and quell the Iraqi insurgency.' As Remnick points out, this reporting has 'stood up over time and in the face of a president whose calumny has turned out to be a kind of endorsement'. Bush reportedly told Pakistan's president, Pervez Musharraf, that Hersh was 'a liar'; after the third of his reports on Abu Ghraib, a Pentagon spokesman announced that Hersh merely 'threw a lot of crap against the wall and he expects someone to peel off what's real. It's a tapestry of nonsense.'

Earlier this year, Hersh turned his attention to Iran: to Bush's desire to bomb it and to America's covert operations there. But while Hersh believes the President would still dearly love to go after Iran, the danger of that actually happening has now passed. Events, not least the sinking of the global economy, have moved on. So he is shortly to write about Syria instead, which he has recently visited. In the dying days of the Bush administration, he says, it is noticeably easier to meet contacts - Cheney, the enforcer, is a lot less powerful - and the information he is getting is good. By coincidence, it was in Syria that he first heard about what was going on inside Abu Ghraib, long before he saw documentary evidence of it. 'I got in touch with a guy inside Iraq during the Prague Spring after the fall of Baghdad, a two-star guy from the old regime. He came up to Damascus by cab. We talked for four days, and one of the things we talked about was prisons. He told me that some of the women inside had been sending messages to their fathers and brothers asking them to come and kill them because they'd been molested. I didn't know whether it was GIs playing grab ass or what, but it was clear that the women had been shamed. So when I first heard about the photographs, I knew they were real. Did I think the story would be as big as it was? Yeah. But was it as big as My Lai? No.' Only a handful of relatively lowly military personnel have so far been punished for their part in the abuse, and Colonel Janis Karpinski, the commander of the Iraqi prisons, was merely demoted (from Brigadier General), in spite of the fact that the Taguba Report, the internal US army report on detainee abuse that was leaked to Hersh, singled her out for blame. 'And John Kerry wouldn't even use it [Abu Ghraib] in his campaign. He didn't want to offend the military, I assume.'

Four decades separate My Lai and Abu Ghraib. You have to ask: wasn't it appalling for him to be investigating US army abuses of civilians all over again? Didn't he think that lessons might have been learnt? Yes, and no. It made him feel 'hopeless', but on the other hand, war is always horrible. In 1970, after his My Lai story, he addressed an anti-war rally and, on the spur of the moment, asked a veteran to come up and tell the crowd what some soldiers would do on their way home after a day spent moving their wounded boys. With little prompting, the traumatised vet described how they would buzz farmers with their helicopter blades, sometimes decapitating them; they would then clean up the helicopter before they landed back at base. 'That's what war is like,' he says. 'But how do you write about that? How do you tell the American people that?' Still, better to attempt to tell people than to stay feebly silent. What really gets Hersh going - he seems genuinely bewildered by it - is the complicit meekness, the virtual collapse, in fact, of the American press since 9/11. In particular, he disdains its failure to question the 'evidence' surrounding Saddam's so-called weapons of mass destruction. 'When I see the New York Times now, it's so shocking to me. I joined the Times in 1972, and I came with the mark of Cain on me because I was clearly against the war. But my editor, Abe Rosenthal, he hired me because he liked stories. He used to come to the Washington bureau and almost literally pat me on the head and say: "How is my little Commie today? What do you have for me?" Somehow, now, reporters aren't able to get stories in. It was stunning to me how many good, rational people - people I respect - supported going into war in Iraq. And it was stunning to me how many people thought you could go to war against an idea.'

As for the troop 'surge' and its putative success, he more or less rolls his eyes when I bring this up. 'People are saying quietly that they are worried about Iraq. This is nothing profound, but by the time the surge got going, ethnic cleansing had already happened in a lot of places. There was a natural lull in the violence. The moment we start withdrawing, and relying on the Shia to start paying members of the Awakening [the alliance of Sunni insurgents whose salaries were initially paid by the US military, and who have helped to reduce violence in some provinces]...' His voice trails off. 'And the big bad bogeyman is Saudi Arabia. There's an awful lot of money going to Salafist and Wahabist charities, and there's no question they'll pour money into the Awakening, and they're so hostile to Shi'ism and to Iran that how can you possibly predict anything other than violence? How do we get out of this? There is no way out. We have a moral obligation to the people of Iraq that goes beyond anything that anyone's talking about. The notion that it's their problem, that we should just leave... I mean, can you believe what we've done to their society? Imagine the psychosis, the insanity, that we've induced.' He stabs the yolk of one of his poached eggs, and sets about his toast like he hasn't eaten in days.

Seymour M Hersh (the M is for Myron) was born in Chicago, the son of Yiddish-speaking immigrants from Lithuania and Poland (he has a twin brother, a physicist, and two sisters, also twins). The family was not rich; his father, who died when Seymour was 17, ran a dry-cleaning business. After school he attended a local junior college until a professor took him aside, asked him what he was doing there and walked him up to the University of Chicago. 'Chicago was this great egghead place,' Hersh says. 'But I knew nothing. I came out of a lower-middle-class background. At that time, everyone used to define themselves: Stalinist, Maoist, whatever. I thought they meant "miaowist". Seriously! Something to do with cats. Among my peers, they all thought I would write the great novel, because I was very quick and cutting. I've just read Philip Roth's new novel [Indignation], and the arrogance of his character reminded me of that certitude. I was always pointing out other people's flaws.' He went to law school but hated it, dropped out and wound up as a copy boy, then a reporter for the local City News Bureau. Later he joined Associated Press in Washington and rose through its ranks until he quit for a stint working for the Democrat senator Eugene McCarthy. Pretty soon, though, he was back in journalism. 'Using words to make other people less big made me feel bigger, though the psychological dimension to that... well, I don't want to explore it.' His wife of 40 years, Elizabeth, whom he describes as 'the love of my life' in the acknowledgements of Chain of Command (they have three grown-up children), is a psychoanalyst. Doesn't she ever tell him about his ego and his id? He looks embarrassed. 'No, no... marriage is... different. When you live with someone you don't... The hardest part for her is when she tells me to take out the garbage and I say: "Excuse me? I don't have time. I'm saving the world."' Later, however, he tells me that journalism, like psychoanalysis, is about 'bringing things into focus'.

He was a broke freelance working for a new syndication agency when he got wind of My Lai. A military lawyer told him that a soldier at Fort Benning, a Georgia army base, was facing a court martial for murdering at least 109 Vietnamese civilians. Hersh rocked up in Benning and went on a door-to-door search, somehow avoiding the officers on base, until he found Lieutenant William L Calley Jr, a boyish 26-year-old otherwise known as Rusty. He asked the former railway pointsman if they could talk, which they did, for three hours. They then went to the grocery store, got steaks, bourbon and wine, and talked some more at the apartment of Calley's girlfriend. Calley told Hersh that he had only been following orders, but nevertheless he described what had happened (it later turned out that soldiers of the 11th Brigade killed 500 or more civilians that morning). Soon after, 36 newspapers ran the story under Hersh's byline. Some, however, did not carry it, in spite of the fact that Calley's own lawyer had confirmed it, among them the New York Times. The scoop caused not only horror but disbelief. Hersh, though, was not to be put off. 'By the third story, I found this amazing fellow, Paul Meadlo, from a small town in Indiana, a farm kid, who had actually shot many of the Vietnamese kids - he'd shot maybe 100 people. He just kept on shooting and shooting, and then the next day he had his leg blown off, and he told Calley, as they medevac-ed him: "God has punished me and now he will punish you."' Hersh wrote this up, CBS put Meadlo on the TV news, and finally the story could no longer be ignored. The next year, 1970, he was awarded the Pulitzer prize.

How does Hersh operate? The same way as he's always done: it's all down to contacts. Unlike Bob Woodward, however, whose recent books about Iraq have involved long and somewhat pally chats with the President, Hersh gets his stuff from lower down the food chain. Woodward was one of those who was convinced that WMD would be found in Iraq. 'He does report top dollar,' says Hersh. 'I don't go to the top because I think it's sorta useless. I see people at six o'clock in the morning somewhere, unofficially.' Are they mostly people he has known for a long time? 'No, I do pick up new people.' But with new contacts he must be wary; there is always the danger of a plant. His critics point to what they regard as his excessive use of unnamed sources. Others accuse him of getting things wrong and of being gullible. A low point came in the Nineties, when he embarked on a book about Kennedy, The Dark Side of Camelot. Hersh was shown documents that alleged the President was being blackmailed by Marilyn Monroe, and though he discovered that they were fake in time to remove all mention of them from his book, the damage to his reputation had already been done - and the critics let rip anyway, for his excitable portrayal of JFK as a sex addict and bigamist. There was also the time, in 1974, when he accused the US ambassador to Chile, Edward Korry, of being in on a CIA plot to overthrow President Allende. Some years later, Hersh had to write a long correction; it ran on page one of the New York Times. As a Jew, his mailbag since 9/11 has also included letters from readers who denounce him as a self-hater (later, at this office, he shows me one of these: its author, an MD with a Florida postcode, accuses him of being a 'kapo' - the kapo were concentration camp prisoners who worked for the Nazis in exchange for meagre privileges).

His supporters, though, believe that his mistakes - and even the wilder allegations he sometimes makes in speeches - should always be put in the context of his hit rate. A former Washington Post reporter, Scott Armstrong, once put it this way. Say he writes a story about how an elephant knocked someone down in a dark room. 'If it was a camel, or three cows, what difference does it make? It was dark, and it wasn't supposed to be there.' Hersh himself points out that, since 1993, he has been up against the stringent standards of the New Yorker and its legendary team of fact checkers. 'By the way, all my inside sources have to deal with the fact checkers, and they do. People find it hard to believe that, I don't know why.' And then there is his editor, David Remnick. 'I never love editors,' he says. 'But David is smart and he has great judgement.' How often does he check in with Remnick? 'I'm sure he would tell you less often than I should. He gets pretty angry with me. Sometimes we have these rows where I won't take his calls. He says no to a lot of stuff - stuff I think the editor would die for! Admittedly, it is not the Seymour Hersh weekly. But sometimes he'll say: "We are not going to publish this kind of stuff 'cos it's frigging crazy."' It was Tina Brown, formerly of Tatler and Vanity Fair, who brought him to the New Yorker. 'What's-her-name... yeah, Tina. She gave me a lot of money, and she said: "Just go do it!" But she used to worry. She'd call me up and say, "I sat next to Colin Powell at dinner last night and he was railing about how awful you are." So I would say, "Well, that's good." And she'd say, "Is it?" And I'd tell her, "Yes, it is."'

Does it worry him that he is sometimes described as the 'last American reporter'? Who is coming up behind him? 'A friend of mine wants to put $5m into a chair for investigative journalism for me, but why would I want to do that? Look, the cost of running my kind of work is very high, and a lot of stories don't even work out. I know a wonderful journalist who works on the internet. I called friends of mine at the Times and the Post. But he hasn't been hired because he would cost a lot of money.' But Hersh is in his seventies (he is a year younger than John McCain, though you'd never know), he can't keep going forever. Or can he? Most reporters start out hungry but somewhere along the way are sated. Not Hersh. 'I have information; I have people who trust me. What else am I going to do? I love golf and tennis and if I was good enough, I'd be professional. Since I'm not, what am I gonna do? Why shouldn't I be energetic? Our whole country is at stake. We have never had a situation like this. These men have completely ruined America. It's so depressing, my business!' Yet he seems chipper. 'No, I'm not chipper. I don't know how to put where I am... I don't take it that seriously. I've been there: up, down, back up. I do a lotta speeches, I make a lotta money, I proselytise.' Does he like making money? 'Are you kidding? I do!'

After we finish breakfast, he takes me to the office. He is eager to put off the moment when he must get on with his Syria piece. The more time he wastes with me... well, the morning will soon be over. Inside he points out a few choice interior-design details - the Pulitzer (it nestles among dozens of other awards), the framed memo from Lawrence Eagleburger and Robert McCloskey to Henry Kissinger, their boss at the State Department, which is dated 24 September 1974, and reads: 'We believe Seymour Hersh intends to publish further allegations on the CIA in Chile. He will not put an end to this campaign. You are his ultimate target.' Then he roots around in a cairn of paper for a while - quite a long while - eventually producing a proof of one of his articles with Remnick's editing marks on it. I've never seen anything so harsh in my life. Practically every other sentence has been ruthlessly disembowelled. 'Yeah, pretty tough, huh?' He also shows me one of his own memos to a contact. It makes reference to the current administration. 'These guys are hard-wired and drinking the Kool-Aid,' it says, deadpan. He laughs. He's getting cheerier by the minute. Soon it will be time for lunch! Now he puts his feet on the desk, removes one training shoe and jauntily waves the sweaty sole of a white sock at me. A couple of calls come in. He is concise bordering on cryptic. Finally an old Times colleague arrives. 'I knew this guy when he had hair!' Hersh shouts as this fellow and I pass in a small area of floorspace not yet covered by books or papers. I'm leaving, but Hersh doesn't get up and he doesn't say goodbye. A breezy salute - and then his eyes fall ravenously on his pal.