NSA had a very hard time while tracking down all Tor users and monitoring their traffic, especially since Tor servers are all over the world. NSA used a zero-day vulnerability in Firefox browser, bundled with Tor, that allowed them to get the real IP address of Tor user. FBI was able to track the Owner of ‘Freedom Hosting’, the biggest service provider for sites on the encrypted Tor network, hosted many child pornography sites. NSA is using web cookies to track Tor user widely. Even you are using the Tor Browser, doesn’t mean that your browser isn’t storing cookies in the system.
Source: https://thehackernews.com/2013/10/nsa-using-browser-cookies-to-track-tor.html