Photograph of the grave of Alexander Litvinenko, Highgate Cemetery, London, United Kingdom (Public Domain via Wikipedia)Alexander Litvinenko took tea with two men at the Millennium Hotel in London on a bright afternoon on the first of November, 2006. That night he began violently vomiting—the beginnings of a three-week episode of radiation poising that would eventually lead to his death. What investigators are still trying to pin down is where that poison came from, and ultimately, who is responsible for the death of the Russian dissident.

Investigators quickly discerned the two men at the hotel were former KGB operatives, and that Litvinenko had ingested a fatal amount of the radioactive element polonium 210, likely manufactured in Russia. But, experts are still trying to determine if polonium has an exact “fingerprint” which would show where it came from.

A new 300-page report released by the British government, this week, documents the forensic investigation into his death, and alleges Litvinenko survived another radiation poisoning only two weeks before in October of 2006.

In the report, forensic experts systematically trace the radioactive poison from the teapot’s spout back to portions of Mr. Litvinenko’s hair that had been cut and tested for polonium after his death. The Health Protection Agency posthumously tested several organs and found Litvinenko had ingested an estimated 4.4GBq—approximately equivalent to 180 times the activity of 1 kilogram of uranium ore.

What experts did not expect to find in the hair analysis was the signs of radiation poisoning, two weeks before the fatal dosage, which they believe likely occurred during a previous meeting with Russian operatives sometime between Oct. 14 and Oct. 25, 2006.

“Tests conducted on Mr. Litvinenko’s hair demonstrated that he had ingested polonium 210 on not one but two occasions,” wrote Chairman Sir Robert Owen, author of the report. “The first dose was much smaller, and had been ingested by Mr. Litvinenko several days earlier.”

Read MorePolonium-210 and The Assassination of Alexander Litvinenko

Experts also address the so-called “fingerprint” theory—that detectable impurities characteristic in every commercially available batch of polonium could be used to trace its origin and its manufacture date. Investigators were hoping that information might secure irrefutable evidence against the murderers. However, the report seems to refute the claim.

“A sample of commercially produced polonium 210 had been analyzed at the Atomic Weapons Establishment (AWE) and found to be entirely pure,” Owen wrote. “There were no signature impurities.”

The report also includes a 260-page “contamination schedule” that lists all the contaminated sites across London. In all, 56 scenes were assessed during the operation, including the hotels where the two men, Lugovoy and Kovtun stayed, as well as the Emirates Stadium, where Lugovoy went to watch Arsenal against CSKA Moscow following the November, 1 meeting with Litvinenko, according to this report.

However, the circumstantial evidence that the two former KGB operatives were the murderers seems hard to ignore. In one of their hotel rooms, radioactive contamination was at the highest levels throughout the investigation.

In addition, experts, looking at CCTV footage, realized that the two men had used a lavoratory during their stay at the hotel, but Litvinenko had not used it at any point. Experts tested the bathroom and found extremely high levels of radiation on the stall door, the sink and the hair dryer—consistent with a secondary transfer, according to the report.

“The open evidence that I have set out above establishes a strong circumstantial case that the Russian State was responsible for Mr. Litvinenko’s death,” Owen wrote in conclusion.

Litvinenko signed his last statement two days before his death. He thanked his adopted Britain, his wife, Marina, and their young son. “You may succeed in silencing one man but the howl of protest from around the world will reverberate, Mr. Putin, in your ears for the rest of your life,” he wrote. “May God forgive you for what you have done, not only to me, but to beloved Russia and its people.”