Following a deadly blast that left at least four dead and 58 houses destroyed, officials in San Bruno, California, have begun an investigation into the causes of the gas explosion. A prime clue appears to be a 28-foot length of pipe that was thrown 100 feet during the blast, and which features an odd assortment of welded parts and a long seam running throughout. Though it’s too early to determine what role it played, these types of marks often point to vulnerabilities, experts say. The pipe will be sent to Washington, D.C. for evaluation. In the meantime, California regulators have mandated that Pacific Gas & Electric, which owns the system, begin its own investigation to ensure that its system is safe. The company was midway through a federally mandated checkup on its most important pipelines when the blast occurred.