Blasts raise questions about pipeline safety

>>> there you new questions about this country's gas pipelines. after a series of explosions including one in pennsylvania that left five people dead. nbc's tom costello has more tonight.

>> oh, my god. this is empty yond belief.

>> reporter: the gas line explosion late thursday night shook the town of hanoverton, ohio, and sent residents fleeing. while no one was injured, a horrific scene in allentown , pa wednesday.

>> i heard an awful bang and thought my roof fell down.

>> reporter: the dead included children and the elderly after an 83-year-old underground gas main exploded damaging or destroying 47 homes and businesses. the cause, still under investigation. in san bruno , california, last september, eight people were killed an 30 homes destroyed after a 30-inch gas transmission line exploded. federal investigators found faulty wells on the pipeline installed more than 50 years ago. the ntsb is in charge of the investigation.

>> and we're monitoring accidents like this one in allentown all over the country, and so we are very concerned about the number of pipeline accidents , and the safety of the infrastructure.

>> reporter: nearly 2.4 million miles of pipeline transport natural gas from well heads to 70 million homes and businesses nationwide. one-third of the transmission line was laid before 1960 . all gas lines are supposed to be inspected at least once a year, and the american gas association insists that if they're maintained, pipe lines should last 100 years or longer, but the american society of civil engineers which gives the country's infrastructure a d, is concerned.

>> while we don't know the cause of these current explosions, it is a sad reflection on the overall condition of our country's infrastructure.

>> reporter: routine maintenance, say the engineers, is critical to keeping any piece of infrastructure running properly. in allentown , the utility company says a routine test came up clean. what triggered the massive explosion?