Core of Corruption is a video documentary documenting the many indications that the alleged Saudi terrorists seemed to be under some type of security service protection while in the USA. Not only did they get special help to get the Visas to get into the US (a consular official who protested to his superiors that these people were terrorist and should not get Visas was ignored), once they were here every time there was a chance they could have been caught circumstances worked out to the hijackers' advantage as the investigators were ordered off the case, not to share their information with other relevant agencies etc. and they were allowed to proceed with their plans apparently unimpeded.
Also covers the role of the Israeli agents apparently shadowing the hijackers, the dancing Israelis on 9/11 celebrating the collapse of the towers, the attack forewarning to the employees of Odigo the Israeli messaging company, the use of wiretapping equipment in US phone systems which was provided to the FBI by an Israeli high-tech company likely with Mossad connections. The Israeli's in the white vans on 9/11 dressed as Arabs. Reports of explosions in the news reports in the immediate aftermath of the 9/11 WTC attacks. - And much more
See also http://coreofcorruption.com