CAIRO - Islamic State's Egyptian affiliate dismissed in an audio message on Wednesday doubts that it had downed a Russian passenger plane over Egypt's Sinai Peninsula, killing all aboard, and said it would tell the world how did so in its own time.The Russian-operated Airbus A321M crashed on Saturday shortly after taking off from the Red Sea resort of Sharm al-Sheikh on its way to St Petersburg, killing all 224 people on board.Sinai Province, an Egyptian group loyal to Islamic State, said in a statement the same day that it had brought down the airliner "in response to Russian airstrikes that killed hundreds of Muslims on Syrian land".The claim was dismissed by Russian and Egyptian officials. Security experts and investigators have said the plane is unlikely to have been struck from the outside and Sinai-based militants are not believed to possess the technology to shoot down a jet from a cruising altitude above 30,000 feet.Russian officials have, however, said the plane probably broke up in the air, leaving open the prospect of some kind of explosion on board.In an audio message posted on a Twitter account used by the group, Islamic State's Egyptian affiliate insisted it was behind the crash. The claim could not immediately be authenticated."We say to the deniers and the doubters: Die from your frustration. We, with God's grace, are the ones who brought it down, and we are not obliged to disclose the mechanism of its demise," the speaker said."So go to the wreckage, search, bring your black boxes and analyse, give us the summary of your research and the product of your expertise and prove that we did not bring it down or how it came down," he said in Arabic."We will disclose the mechanism of its demise at the time that we want and in the way that we want."