MELVINDALE, Mich. -

No one was hurt Saturday when the Marathon Refinery caught fire in Detroit.

Private and public firefighters contained the blaze in about an hour and 1/2. The systems in place worked as designed.

Still, many wonder on Monday what happened and why was a neighborhood evacuation necessary?

Read more: Probe underway after Detroit refinery fire

The smoke came from a fire in a 45,000-barrel sour water tank behind the Marathon fence on Schaeffer Road. Sour water is a byproduct of oil refining that gets treated and cleaned.

Listen: 911 calls from Marathon Refinery fire

On Monday, Marathon's fire cleanup team poured foam onto the tank to lessen the odor as they drained contents from the mangled tank and other tanks. Local 4 has learned it was more than a slew of dirty water inside when it burned.

Inside the tank were benzine and hydrogen sulfide covered by about 4 feet of diesel fuel used to contain those chemicals.

It was the diesel fuel that burned. Benzine and hydrogen sulfide can penetrate skin, which is why they pulled the trigger on a 3,000-person evacuation.

Ken Duncan has lived about 1/2 mile from the refinery for 10 years. He doesn't wait for permission. If there is trouble, he jumps.

"When you know it's time to go, there is nothing you can do about it, you just leave," Duncan said.

The tank will have to be rebuilt. The question is when. Right now, Ashland/Marathon is saying they don't know when they're going to rebuild it because they still have to figure out why it burned.

Such tanks don't burn like this very often, so there are a lot of questions still to be answered.