However , an investigation by the town of Billerica eventually determined that the source was a local company that produced surgical and medical materials .
Initially , it was believed that explosives used in road and building construction were the source of this pollutant .
An investigation by the town of Billerica eventually determined that the source was a local company that produced surgical and medical materials .
Initially , it was believed that explosives used in road and building construction were the source of this pollutant .