U.S. overhauling intel access in bid to prevent another Snowden ASPEN, Colorado (Reuters) - The United States is overhauling procedures to tighten access to top secret intelligence in a bid to prevent another mega-leak like the one carried out by former spy agency contractor Edward Snowden, a top Pentagon official said on Thursday. Deputy Defense Secretary Ashton Carter told a security forum that the government was already moving to better isolate intelligence so that all of it isn't accessible in one place, and to implement a "two-man rule" - similar to procedures used to safeguard nuclear weapons. When are we taking countermeasures? ... The answer is now," Carter told the Aspen Security Forum in Colorado. This failure originated from two practices that we need to reverse. U.S. intelligence agencies conducting a forensic review of the activities of Snowden, a former National Security Agency contractor, are close to pinning down the extent of the classified documents he accessed and the means by which he removed materials from a secure environment, according to intelligence and security officials close to the investigation. Carter declined to delve into details, saying the assessment was still ongoing. But he added: "I can just tell you right now the damage was very substantial." Snowden has provided documents about secret U.S. and British eavesdropping programs to Britain's Guardian newspaper, the German magazine Der Spiegel and the Washington Post. He also made allegations about U.S. eavesdropping on Chinese targets to the Hong Kong-based South China Morning Post. The 30-year-old American who has had his U.S. passport revoked, is stuck in the transit area of Moscow's Sheremetyevo airport and has applied for temporary asylum in Russia. Senator Dianne Feinstein, who chairs the Senate Intelligence Committee, said last month that U.S. officials advised her that Snowden had roughly 200 classified documents. However, American officials and others familiar with Snowden's activities say they believe that at a minimum, he acquired tens of thousands of documents. Current and former U.S. officials say that while authorities now think they know which documents Snowden accessed, they are not yet sure of all that he downloaded. Snowden was adept at going into areas and then covering his tracks, which posed a challenge in trying to determine exactly what materials he had accessed, officials said. Carter partly blamed the emphasis placed on intelligence sharing in the wake of the September 11, 2001 attacks, which allowed someone like Snowden to access so many documents at once. "We normally compartmentalize information for a very good reason, so one person can't compromise a lot," Carter said. Loading everything onto one server ... It's something we can't do. Because it creates too much information in one place. He told reporters that the efforts to create more barriers to accessing information were under way, as were efforts to create a two-man rule for some operations. Asked where the two-man rule was being put into effect, Carter said: "Everywhere where there are system administrators who had elevated type access, those procedures are (being adopted)." Former and current U.S. officials told Reuters that a massive overhaul of the security measures governing such intelligence would be extremely expensive. (Additional reporting by Tabassum Zakaria and Mark Hosenball. Editing by Christopher Wilson)