An article says that they used that in the London Bombings on 7-11 last year. It's strange they say the police "theorized" -- don't they know for sure?
"After the July 7 suicide bombings in London last year, British police theorized that acetone peroxide triacetone triperoxide (TATP) was the "home-made" explosive used by the plotters."
http://www.cnsnews.com/news/viewstory.asp?Page=%5CForeignBureaus%5Carchive%5C200608%5CINT20060811c.html
Maybe they do not wish to lock themselves into only one explosive. This one is said to be easily concocted and like recipes for meth, explosives can likely be made from household and commercial use products.