Just after midnight on April 2, 2010, a massive fireball lit up the sky over Anacortes, Washington.

A heat exchanger at a local oil refinery had ruptured, causing highly flammable gases to ignite.

When people asked me how much of him was burned I'd tell them it's easier for me to tell you what wasn't burned ... This accident could've been prevented ... I want people to remember what happened. I don't want it to happen again. So if I have to - if I have to tell my story till the day I die, I will.

Shauna Gumbel, mother of operator Matt Gumbel who died after the Anacortes explosion

Seven workers were inside the refinery's unit when it exploded. Three of them died instantly; the other four died later from their injuries.

What happened that night and the months and years since has largely been out of the public eye.

State and federal investigators concluded that the refinery's owner, the Tesoro Corporation, had ignored signs of deterioration and other safety hazards that, if dealt with, could have prevented any loss of life.

"The equipment there we found had been weakened by nearly 40 years of exposure to extreme chemical and physical stresses that resulted in cracks in the metal that could have been discovered in our judgment with proper testing and required testing that had not been done in more than 10 years," says Michael Silverstein, who oversaw the investigation into the explosion for Washington state's Labor and Industries Department.

It was the single largest industrial accident in the state of Washington ever. But the company is still refusing to pay out any penalties, insisting it didn't do anything wrong.

In an exclusive investigation, produced in collaboration with The Center for Public Integrity, Fault Lines reports on the aftermath of the Anacortes explosion, and ongoing efforts by Tesoro to avoid any blame.

During the investigation into the Anacortes explosion, Fault Lines requested responses from the Tesoro Corporation in two separate emails. You can read the company's full response here.

Source: Al Jazeera