Alexander Litvinenko, a former KGB spy, photographed at his home in London in  2002, left, and in a hospital bed in London on Nov. 20, 2006. He died Nov. 23, 2006. (Alistair Fuller/AP; Reuters)

LONDON — The killing of former KGB spy Alexander Litvinenko began with a poison-laced cup of tea and ended 25 days later with an excruciating death in a London hospital.

Nearly eight years have passed since then, and much about Litvinenko’s killing remains a mystery. Who exactly killed the spy-turned-whistleblower? More important: On whose orders, and why?

On Tuesday, there was new and unexpected hope that some of those mysteries will finally be solved.

The hope came in the form of a U-turn from the British government, which has long blocked a full public inquiry into Litvinenko’s death but on Tuesday said it would allow the investigation to go forward.

Litvinenko’s widow has alleged that British authorities didn’t want anyone digging into the case because they feared that it would upset Russian President Vladimir Putin. The British government acknowledged as much last year, saying that “international relations” had been a factor in decision-making around the case.

But Putin’s wrath is not such a concern anymore, now that British Prime Minister David Cameron is calling for Europe to hit Russia hard with new sanctions after last week’s downing of Malaysia Airlines Flight 17 in eastern Ukraine. Cameron and others in the West have blamed the crash on Russian-backed rebels.

British officials stressed  Tuesday that the timing of the Litvinenko decision and the push to punish Moscow were entirely unrelated.

But few in London believed that.

Details about the killing of Litvinenko have long been seen here as deep, dark secrets with the potential to embarrass both London and Moscow. Litvinenko had fled to London in 2000 after becoming an outspoken critic of his former employer, the Russian domestic intelligence service. His widow has said that he was working for Britain’s foreign intelligence service, MI6, at the time of his death at age 43 from exposure to radioactive polonium.

The two Russians who sipped tea with Litvinenko at a posh London hotel before he fell ill have been named as prime suspects in his killing. But they deny involvement and remain in Russia.

Until now, the investigation has been prohibited from considering evidence that the Russian government played a role in the killing. But after Tuesday’s decision, that will change.

“It is more than 7 years since Mr. Litvinenko’s death, and I very much hope that this inquiry will be of some comfort to his widow Mrs. Litvinenko,” Home Secretary Theresa May said in a written statement to Parliament.

Marina Litvinenko, who has long campaigned for the facts of her husband's killing to be brought to light, celebrated Tuesday’s about-face, saying she was “relieved and delighted.”

“No matter how strong and powerful you are,” she said in a written statement, “truth will win out in the end.”