A former Russian spy who died in an apparent poisoning signed a statement on his deathbed blaming Russian President Vladimir Putin for his death, friends said Friday. The government said Alexander Litvinenko's death was linked to a radioactive substance in his body. Home Secretary John Reid, the country's top law-and-order official, said experts had been called in to search for "residual radioactive material" at a number of locations as police investigate the cause of Litvinenko's death. Reid said in a statement that Litvinenko's death Thursday night was "linked to the presence of a radioactive substance in his body." Litvinenko's statement, read to reporters outside the hospital where the ex-spy died late Thursday, accused the Russian leader of having "no respect for life, liberty or any civilized value." "You have shown yourself to be unworthy of your office, to be unworthy of the trust of civilized men and women," Litvinenko said in a statement read by his friend Alex Goldfarb. "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." Goldfarb said Litvinenko had dictated the statement before he lost consciousness on Tuesday, and signed it in the presence of his wife, Marina. Putin's government has strongly denied involvement. "The allegations against Russia in this respect are nothing but nonsense," said Kremlin spokesman Dmitry Peskov before Litvinenko's statement was made public. "It's so silly and unbelievable that it's not worth comment," Peskov said in Helsinki, Finland, where Putin is attending a summit with European Union leaders. "Now the case will be investigated by relevant British services and we hope that those who are standing behind this case will be brought to justice," he added. Litvinenko, a vociferous critic of the Russian government, suffered heart failure late Thursday after days in intensive care, London's University College Hospital said. Doctors said the cause of his illness remained a mystery, discounting an earlier theory that the 43-year-old father of three had been poisoned with the toxic metal thallium or a radioactive substance. British health authorities were to hold a news conference later Friday. "As part of this investigation, the police have called in expert assistance to search for any residual radioactive material at a number of locations," Reid said. Friend Andrei Nekrasov, who spoke to Litvinenko just before he lost consciousness, said the former KGB agent had accused Russian intelligence services of poisoning him. Friends said Litvinenko had been on a quest to uncover corruption in Russia's Federal Security Service, or FSB, and unmask the killers of another trenchant critic of the Putin's government, investigative journalist Anna Politkovskaya. "He was completely convinced it was the FSB. There was no doubt in his mind who it was," Nekrasov told The Associated Press. Nekrasov said Litvinenko had told him: "The bastards got me, but they won't get everybody." Litvinenko told police that he believed he had been poisoned on Nov. 1, while investigating the slaying of Politkovskaya. His hair fell out, his throat became swollen and his immune and nervous systems were severely damaged. He was transferred from a north London hospital to University College Hospital on Nov. 17 when his condition deteriorated. London's Metropolitan Police confirmed that Litvinenko had died at 9:21 p.m. (2121 GMT) Thursday. It said anti-terrorist officers were investigating the matter as "an unexplained death." "It was an excruciating death and he was taking it as a real man," Litvinenko's father, Walter, told reporters Friday. "This regime is a mortal danger to the world," he added, his voice choked with emotion. Nekrasov told the AP that Litvinenko's father, his wife and his 10-year-old son, Anatoli, were by his side when he died. He said the former spy had begun to lose consciousness on Tuesday. "It was a darkened room, and he would open his eyes now and again. We were encouraging him, telling him that he would survive," Nekrasov said. "It was so heart-rending. His son was just in a state of shock. He didn't know what to make of it. The family just huddled in a corner of the hospital _ it was terrible to look at." Nekrasov said Litvinenko believed he had been targeted by the Kremlin because he had threatened to uncover embarrassing facts. "The only logic is revenge, they consider him an enemy _ every week he was in Putin's face, he was a tireless critic of Putin's regime ... He had a mission to uncover what he felt were crimes his former colleagues had committed," Nekrasov said. Litvinenko worked for the KGB and its successor, the FSB. In 1998, he publicly accused his superiors of ordering him to kill tycoon Boris Berezovsky and spent nine months in jail from 1999 on charges of abuse of office. He was later acquitted and in 2000 sought asylum in Britain, where Berezovsky is now also living in exile. On the day he first felt ill, Litvinenko said he had two meetings, the first with an unnamed Russian and Andrei Lugovoy, an-KGB colleague and bodyguard to former Russian Prime Minster Yegor Gaidar. Later, he dined with Italian security expert Mario Scaramella to discuss the October murder of Politkovskaya. Scaramella said he showed Litvinenko an e-mail he received from a source naming the killers of Politkovskaya, and naming other targets including Litvinenko and himself.