“Nobody knows who stabbed who, and we can't even get up to business class right now 'cause nobody can breathe.” - Betty Ong, Flight 11

Not necessarily explosive, but tactical chemical attacks have been a known capability for a while, and part of anti-terrorist training long before 2001. Something truly sophisticated from a superpower state would likely escape detection, (and therefore likely implicate such a state,) but the Tokyo Sarin attack by a cult in 1995 involved big bags of liquid.