Though it did not strike anyone, four bystanders were injured when concrete, glass and other debris was thrown into the air. They were treated at nearby hospitals for various cuts, bruises and burns and released. Initial damage estimates from the blast were $750,000 to $1 million.
The explosion, which happened just before 10:30 a.m., was caused by intense steam pressure that built up inside the water heater, investigators said.
"The pressure-relief valve had been capped, and the tank was partially drained of water," said King County Fire Battalion Chief Doug Hudson. "It built up pressure and exploded." As of this writing, investigators were unsure if the valve was capped on purpose. Plumbers had been at the building Thursday, but it was not clear if they worked on the water heater.
The building in Burien, about 10 miles south of Seattle, housed a Mexican restaurant, a video store and a grocery store. The building has since been condemned.