NatGasFag here. Posters are saying natural gas relief valves are failing. A search of natural gas relief valves are what we call "regulators" in the industry. They attach to the meter manifold at the house and are spring loaded mechanical devices that reduce the pressure of the gas coming from the mainline to an acceptable usable lower pressure. They are always located on the outside of the house and usually up against the house. They could be individually tampered with using some basic tools but there's no way for them to fail en masse simultaneously. Taking the spring out of them would push a large pressure of gas inside the house and snuff the pilot lights and fill the basement and then the house with gas. If you were home you would smell it. If you were away the house would become a bomb and when your fridge kicked on, the house would blow

NatGasFag is not good at posting and sucks at responding. Regulators are entirely mechanical. Most Natural Gas equipment is made in the United States (to my knowledge) by only a few companies like Honeywell, Singer, American Meter, Itron. If they were failing it was most likely sabotage. If there was a manufacturing defect, why would it manifest all at once?

NatGasFag here. If the houses were on a "low pressure" system, there may have been no regulators on the manifolds and the manifolds could be located inside the basement. If someone then went to a regulator station which are usually little brick buildings scattered about town where they reduce higher pressure transmission lines to low pressure distribution lines, and opened up a valve to blast gas into a low pressure line that is not designed for that, you would have the same effect as a failed regulator I suppose. But that should have set off some serious monitoring alarms