LONDON (AP) — The European Court of Human Rights on Tuesday backed the conclusion of a British inquiry that Russia was responsible for the killing of Alexander Litvinenko, who died in London in 2006 after drinking tea laced with a radioactive material.

A former agent for the KGB and the post-Soviet successor agency FSB, Litvinenko defected from Russia in 2000 and fled to London. While in Britain, Litvinenko became involved in exposing corruption and links to organized crime in the Russian intelligence service.

He fell violently ill on Nov. 1, 2006, after drinking tea with two Russian men at a London hotel, and spent three weeks in the hospital before he died. His tea was found to have been laced with radioactive polonium-210.

The British inquiry concluded in early 2016 that Russian agents Andrei Lugovoi and Dmitry Kovtun had killed Litvinenko, and that President Vladimir Putin had “probably approved” the operation.

Litvinenko's widow, Marina, took the case to the Strasbourg-based court.

The European court backed the British conclusion in its verdict on Tuesday but rejected Marina Litvinenko's claim for “punitive” damages.

"The Court found in particular that there was a strong prima facie case that, in poisoning Mr. Litvinenko, Mr. Lugovoi and Mr. Kovtun had been acting as agents of the Russian State,” it said.

It also noted that the Russian government had “failed to provide any other satisfactory and convincing explanation of the events or counter the findings of the U.K. inquiry.”

Both Lugovoi and Kovtun deny any involvement in the killing.

Kremlin spokesman Dmitry Peskov told reporters Tuesday that the ruling was unsubstantiated.

“We’re not ready to take such rulings on board,” he said.