Explosion at unfinished Tokyo subway

An unfinished subway experiences a brief explosion, initially causing public panic as people suspect a bomb from the sky. However, it is later revealed that the explosion was due to a gas leak from a main into the subway, which was still under construction.

How did people initially react to the explosion?

What caused the explosion in the subway?

What was the actual source of the explosion?

What type of gas was involved in the explosion?

What was the state of the subway at the time of the explosion?