MOSCOW (AFP) - An Islamist group claimed Friday it caused the disaster at Russias largest hydroelectric plant, as Prime Minister Vladimir Putin visited the site and acknowledged 75 people were killed in the tragedy. Russian investigators denied the claim by Riyadus Salikhiin, a militant group with roots in Chechnya, that it had hit the power station on Monday as part of a new campaign of economic war in Russia. The claim, posted on rebel website, also said the group had carried out a suicide bombing at a police headquarters in Russias Ingushetia region that killed two dozen people the same day as the dam disaster. There was no way to check the veracity of the claims, which the website said it received by email. Officials insisted the dam tragedy was caused by a technical fault and there was no evidence of attack. Russian security service explosives experts have found no trace of explosive material at the site of the accident, said Vladimir Markin, spokesman for the investigative committee of state prosecutors. Various theories for the cause of the accident are under investigation. However there has been no confirmation of the theory of a terrorist act, Markin said, according to news agencies. The rebel statement said militants had carried out an attack at the power plant in southern Siberia by placing an anti-tank grenade in the turbine hall on a timer and later detonating it. Praise to Allah, on August 17 an act of sabotage was carried out at the Sayano-Shushenskaya hydroelectric plant that had been carefully thought out and prepared for a long time, the statement said. Putin, on a visit to the plant Friday, indicated that 75 people had been killed in the accident, which officials say took place when a massive flood engulfed the main turbine hall. We see what has happened, we know what has happened. Theres no need to pretend that theres something unclear about this to someone, Putin said, after officials announced 47 people were killed and 28 missing. Steel can be restored but people will not be brought back. This is the biggest tragedy, Putin said. The rebel statement said that also on Monday an operation was carried out to destroy the occupying police forces in Nazran, the main city of the increasingly violent Caucasus region of Ingushetia. It said a suicide bomber broke through the gates of the police headquarters in a car and then set off his explosives. According to the latest official toll, 25 people were killed in the Nazran attack. Investigators did not immediately comment on the veracity of the Islamist groups claim of responsibility. The dam disaster and the Ingushetia attack came amid mounting violence in Russias overwhelmingly Muslim northern Caucasus region, where Islamists are waging an insurgency against the pro-Kremlin local authorities. In the latest Caucasus violence on Friday, four policemen were killed in the capital of Chechnya in a double suicide bombing carried out by attackers riding bicycles, Russian officials said. Chechnyas leader Ramzan Kadyrov praised the slain police who he said had given up their lives to prevent even greater casualties, the Interfax news agency reported. Riyadus Salikhiin is on a blacklist of groups, maintained by the US State Department, which are not allowed entry into the United States because of their links to terrorist activities. In June, the group claimed responsibility for a bomb attack that badly injured the president of Ingushetia, Yunus-Bek Yevkurov. Riyadus Salikhiin calls itself a martyrs brigade and was previously led by Shamil Basayev, the notorious Chechen warlord who masterminded the Beslan school hostage-taking of 2004 and was killed in 2006. More recently the group has said that its commander is Chechen guerrilla leader Doku Umarov, head of the self-proclaimed Caucasus Emirate which wants to establish an Islamic state across Russias North Caucasus. In its statement Friday, the group said it had decided earlier this year at a meeting of its leadership to activate an economic war against Russia on its territory. To carry out this task, sabotage groups were set up and sent to numerous regions of Russia with the aim of carrying out acts of industrial sabotoge.