Cairo - The Islamic State group on Wednesday insisted it brought down a Russian plane that crashed in Egypt's Sinai Peninsula, providing no new details but challenging sceptics to prove otherwise.The jihadist group had claimed on Saturday that it downed the Airbus in Sinai, where its Egypt affiliate is based, but provided no details, prompting scepticism about its involvement.All 224 people on board the flight bound for Saint Petersburg, mainly Russian tourists, were killed.Experts say the probe into the crash will take time as they analyse recovered black boxes and wreckage that was strewn across a wide area.In an audio statement posted on social media sites on Wednesday, ISIS said it would announce the details of the alleged attack when it chooses."We are under no obligation to explain how it came down," ISIS said in the statement, posted a day after it released a video showing its fighters in Iraq celebrating the incident."Bring the wreckage and search it, bring your black boxes and analyse them, and tell us the results of your investigation," a man said in the recording."Prove that we didn't bring it down, and how it came down. We will detail how it came down at the time of our choosing."The group claimed that the plane was brought down on the 17th day of the month of Muharram in the Muslim lunar calendar, the first anniversary of the Egyptian affiliate's pledge of allegiance to ISIS.