There exist other material that go boom beyond gas. Just to take a random pick, maybe a truck with industrial chemicals was parked near. The majority of goes boom but a chunk get thrown away and just later goes boom too. Double explosion, but no detonated explosives.
Not saying its likely, but we really should start listing on what police say first before jumping directly to bombs.
At this point things seem to be pointing towards bombs. Police have reportedly recovered 2 or 3 undetonated devices (and set off a controlled explosion for one of them [1]). No news of who or why though.
Nobody is pointing "blame." But we can discuss who the likely suspects would be.