Dr. Crockett Grabbe of the University of Iowa has had a new paper published at the Journal of 9/11 Studies. He studies visual evidence for explosive activity during the destruction of the WTC towers, and follows closely on Kevin Ryan's paper, which studies similar subject matter:

Direct Evidence for Explosions: Flying Projectiles and Widespread Impact Damage by Dr. Crockett Grabbe

High Velocity Bursts of Debris From Point-Like Sources in the WTC Towers by Kevin Ryan

INN News Report interviews Dr. Grabbe (April, 2007)
http://video.google.com/videoplay?docid=2438958231402017959&hl

(Don't forget the "Letters" section, which is updated periodically;
http://www.journalof911studies.com/letters.html )