They used a peroxide two part solution. Exactly what the other part was they haven't said. I gather there are several things that could have been used and I am sure all this is common knowledge of a lot of terrorists.
It's not that easy to obtain peroxide in high concentration, though it is possible for industrial usage. The stuff you buy in the drugstore is 97% water, and even the kind used for hair coloring is at most 12% active (88% water).
But I bet peroxide will now go on the list of substances for industries to track.
Simple to make and use. Especially simple if no ignition source is needed.
Oooh..
Don't say peroxide.
I had a bad experience at work about seven years ago with peroxide (10%) and H2SO4.
To this day, I'm the only one who knows about it.
Shhhh........
Acetone (nail polish remover) perhaps? See TATP, known among sand nazis as Mother of Satan because of its propensity to cause "work accidents". But a work accident high over the North Atlantic might work just as well as proper explosive preparation.