JACKSONVILLE, Fla. -- Witnesses to last week's fatal chemical plant explosion described what they saw as very loud, very violent and even as bomblike.

The blast shook homes for miles around, making people wonder what had blown up on Jacksonville's Northside.

Dispatcher: Do you actually see fire now?

Caller: Uh. Ma'am there is a good-sized fire.

Dispatcher: OK.

Caller: I see dark smoke.

Dozens of people called 911. Each witness relayed that something was very wrong on Jacksonville's Northside last Wednesday.

Caller: I'm out here on Black Hammock Island and I was outside and heard something that sounded like a bomb.

Caller: I'm out on my boat with my son and there's been a really big size explosion on the corner of Alta Drive and Faye Road.

Dispatcher: Alta Drive and Faye? Can you tell what it is?

Caller: I have no idea. I'm in my boat and it is a good-size mushroom cloud.

Dispatcher: Is it possibly a transformer or anything or can you tell?

Caller: Oh no, no, no, this is way bigger than a transformer. It shot something; I'm talking thousands of feet in the air.

Dispatcher: But does it look like it could be a building or anything?

Caller: Ma'am, I'm only looking over the trees but I can uh -- I'm just letting you know it was a very loud very violent explosion.

What they saw was later declared a catastrophic industrial accident -- T2 Laboratories exploded in a mushroom cloud, killing four people and injuring 14 others.

Robert Scott Gallagher, co-owner of the facility that makes fuel additives and solvents, was among the dead. Also killed were Charles Budds Bolchoz, 48; Karey Renard Henry, 35; and Parrish Lamar Ashley, 36

Investigators charged with finding out what caused the T2 Laboratories explosion told U.S. Sen. Bill Nelson that something went wrong while octane booster was being mixed at a Northside chemical plant.

"This octane booster ... is a fairly stable product, but to get to it, you use these highly energetic chemicals," Nelson said Friday. "A lot of heat is generated and a chemical process starts that can get in a runaway condition."

On Wednesday, family and friends gathered at a Northside church to say goodbye to Bolchoz. The service was held at St. Patrick's Catholic Church. His family said Bolchoz would be buried in South Carolina.

Officials from the Chemical Safety Board, OSHA, and ATF, along with representatives from state and local agencies continue to look into the fatal explosion and fire.

Investigators with the Chemical Safety Board said it could take as many as six months to learn the official cause of the explosion.