I recall that boxcutters were being found on other flights for several weeks after 9-11.
http://archives.cnn.com/2001/US/09/23/inv.investigation.terrorism/
“Charles Miller, a U.S. Justice Department spokesman, told CNN some box cutters were found on other planes besides the four hijacked ones, but he did not provide any details.”
And...
http://www.dailymail.co.uk/news/article-200951/Box-cutters-US-planes.html
I have heard that “20 planes” number before. I don’t doubt anything. Seems interesting that 3 Hezbollah members were arrested yesterday at an airport in Mexico.