Tomorrow fire investigators plan to return to the scene of today's deadly explosion in Campbell Industrial Park, where one man was killed and three others were injured.

The explosion happened at 1:30 this afternoon at an oil and water treatment plant on Komohana Street.

It also affected a neighboring business.

The force of the explosion sent objects and people flying.

As emergency crews worked to make sure everything and everyone was o-k.

"We know that there was a rupture of a tank," said Honolulu Fire Capt. Terry Seelig.

Workers gathered together grieving -- in disbelief.

"It's very hard especially when it comes to a person's life. It's very hard and difficult for us here too as well. This impacted our employees tremendously," said Ray Luavasa, Philip Services Hawaii Health & Safety Manager.

Luavasa says two employees and four contractors were working at this Philip Services Hawaii plant when this oil storage tank exploded.

It was too close of a call for contractor Randall Ikuta.

"The fireball was so big. Little burnt, my hairs got burnt," said Ikuta.

The force of the explosion sent one worker flying into the neighboring property.

"That's where he came to rest near or inside of the pickup truck," said Emergency Medical Services Spokesman Bryan Cheplic.

Paramedics pronounced the 23-year old man dead shortly after they arrived on scene.

The man worked for Panco and was working here as a contractor.

Three other people were taken to the hospital in stable condition -- two were contractors, the other worked for neighboring business Bonded Materials.

"Why the explosion happened we don't know. That will be under investigation," said Seelig.

Philip Services Hawaii is mainland-owned.

The company has two locations in Campbell Industrial Park.

This is an oil and water treatment plant -- they take oil from ships, cars and underground storage tanks and process it for re-use.

Company officials say the huge tank that exploded contained oil that hadn't been re-processed yet.

The force of the explosion hurled the tank and a metal grate known as a catwalk over the wall, into the property of Bonded Materials.

"All the regulatory agencies came done their investigations and we're still under investigation too as well," said Luavasa.

The Medical Examiner's Office has not yet released the name of the man who died.

Fire officials say it may be a couple days before they know what caused the explosion.