LONDON » It has consumed more than eight years of maneuvering, obstruction and a widow’s dogged legal campaign, fought often on a shoestring. But, finally, on Tuesday, a public inquiry is set to begin its quest for an answer to the question that has driven the whole process: Why did Alexander V. Litvinenko have to die?

On Nov. 1, 2006, Litvinenko, a former officer of the Soviet KGB in self-exile in London and a vocal critic of President Vladimir V. Putin, sipped tea from a poisoned pot, took sick and died 22 days later. Only after his death did British scientists confirm that the poison was Polonium 210, a rare isotope manufactured mostly in Russia.

From his deathbed, Litvinenko accused Putin of responsibility for ordering his murder, a charge the Kremlin leader has always denied. But when Robert Owen, an eminent British judge, opens the inquiry Tuesday, it will be to start months of hearings that will paint a detailed canvas of Litvinenko’s life and death, and try to determine where responsibility lies among a flamboyant cast of potential players ranging from Russian state agencies, to British spies, to Spanish mobsters.

For what was almost a perfect crime, the inquiry will offer an imperfect mirror. Reflecting the insistence of British officials that disclosure of some testimony would harm national security, many issues will be addressed in secret, and even parts of the judge’s final report will not publicly allude to some findings.

But for Litvinenko’s survivors, notably his widow, Marina, 52, a former ballroom dancer and aerobics instructor, and his son, Anatoli, 20, a student of politics, the inquiry will very likely be a watershed. Perhaps, Marina Litvinenko said in one of a series of interviews, the inquiry will yield "a kind of finishing of what I have been doing for the last eight years."

"I will have the official verdict," she said. In a way, the surprising element is that it has got this far.

Initially, the Litvinenko’s death drew British protests. Diplomats were expelled from London and Moscow in tit-for-tat gestures reminiscent of the Cold War. Britain froze formal cooperation with Russian intelligence. But then, in 2010, the government here changed, and, like President Barack Obama, a new British prime minister, David Cameron, sought better relations with Putin.

"We had the Litvinenko affair; we imposed the sanctions," said Anthony Brenton, Britain’s ambassador to Moscow from 2004 to 2008. "The Russians didn’t crumble. Time rolled on and we have serious other interests in Russia."

In British government circles, he said in an interview, "as the Litvinenko affair began to fade into history, so the argument for getting past the Litvinenko affair got stronger and stronger."

For its part, Moscow resisted the effort to illuminate a death that some British diplomats blame squarely on the Kremlin. The inquiry will also consider whether the poisoning stemmed from other parts of Litvinenko’s shadowy life after his flight from Russia, when he worked variously for the former oligarch Boris A. Berezovsky, Britain’s MI6 intelligence agency and Spanish investigators seeking to expose organized crime.

Only days before the start of the inquiry, moreover, an audiotape unearthed by The Daily Telegraph newspaper was said to have been recorded by Litvinenko exactly one year before his death and purported to trace direct links between Putin and a fugitive Ukrainian crime boss suspected of selling arms to al-Qaida.

Citing constitutional constraints, Russia has refused to extradite one of its citizens, Andrei K. Lugovoi, to be questioned in Britain about murder charges. Lugovoi and a second Russian citizen, Dmitry V. Kovtun, were both present in the Pine Bar at the Millennium Hotel in central London’s Grosvenor Square when Litvinenko was poisoned, but both men have sought to flip the narrative, accusing him of trying to poison them.

Russian prosecutors have sought to delay the inquiry, lodging 1,300 pages of Russian-language documents to bolster their own case concerning Lugovoi and Kovtun. Finally, Moscow’s prosecutors, once accorded the status of "core participants" with special privileges at the inquiry, withdrew as the Kremlin dismissed Owen’s scrutiny as a sham because of its provisions for secret hearings.

For Marina Litvinenko, who endured numerous ups and downs over the years, 2013 was "the most difficult year."

Berezovsky, once her main financial backer, had died in still ambiguous circumstances. Citing national security and other concerns, two senior British Cabinet ministers, Foreign Secretary William Hague and Home Secretary Theresa May, had gone to significant lengths to restrict – or even block – public scrutiny of Alexander Litvinenko’s death.

For a single mother raising a teenage son, Marina Litvinenko faced a tough call: if she lost one last battle to secure a public inquiry, she would face a bill from government lawyers for $80,000 that she did not have. She was relying, as she still does, on a legal team working for her pro bono.

"I had three days to decide: to go or not go" ahead with an appeal, she said. "Nobody could tell me what to do. I am not a gambler. I don’t play the casino. I try to have something reasonable."

In the end, she concluded that she had to take the risk. "This is my personal cause. How can you rest or do nothing without feeling you have done everything you can?"

Finally, in February last year, senior judges ordered May to reconsider her opposition to a public inquiry and, in July, as Russia drew ever greater Western opprobrium over the crisis in Ukraine, the global pendulum swung in Litvinenko’s favor. May dropped her opposition to an inquiry.

In a way the inquiry will offer a denouement to a drama that had been building since 1993, when Litvinenko met her future husband in Moscow at a friend’s birthday party.

As her husband and his mentor, Berezovsky, cast themselves as whistle-blowers against corruption in the FSB, the domestic successor to the KGB, they made more and more enemies, including Putin, himself a KGB alumnus. Their denunciations of the FSB soon rebounded against them.

Suspended from the FSB, Alexander Litvinenko was tried in early 1999 on charges related to allegations that he abused the powers of his office. When he was acquitted, masked officers in camouflage fatigues rearrested him on extortion charges. A year later, encouraged and soon followed by Berezovsky, he fled Russia for Britain, where both men secured asylum.

Her husband’s flight into self-exile plunged Marina Litvinenko into a world more familiar to readers of spy thrillers. "Sasha called me and told me I have to go to Turkey and meet him," Marina Litvinenko said, using the shortened name by which her husband was widely known. "It was the first time he told me: ‘Marina, prepare yourself not to go back to Russia.’ It was very devastating for me. I just could not believe it."

Six years later, at about the same time as her husband was sipping polonium-laced tea, Litvinenko was home preparing her husband’s favorite chicken dish to celebrate the anniversary of their arrival in Britain.

Just weeks earlier, the family had secured British citizenship, and Alexander Litvinenko had told his son that the British crown "protects us, guards us." But his wife could hardly have known that her life henceforth would be devoted to trying to illuminate the dark saga of her husband’s death.

"This kind of activity is like a marathon," she said. "You never know when you will need to be strong."

Alan Cowell, New York Times