Fire officials in Defiance, Ohio say human error caused that powerful explosion and fire Thursday at a bio-diesel processing plant.

Employees inadvertently left a manhole cover open on a storage tank holding glycerin, and a spark from an electric motor ignited the methanol vapors that escaped.

Neighboring homeowners and workers are trying to settle their nerves a day after the blast.

Home video obtained by Indiana’s NewsCenter captured a second, smaller explosion at the American Ag Fuels Plant.

Broken glass was scattered at nearby C.R. Hunt Incorporated, remnants of the initial, more powerful blast.

Clifford Hunt/Business Damaged by Blast: " The big 8 foot high windows, it blew those out. It took out two overhead doors."

Julia Ryan, who was in the office, ran out to look when she heard and felt the concussion.

Julia Ryan/Eyewitness: " It's hard to describe. I guess the thing that comes to my mind is like the Hindenburg when that went down."

Three workers inside the bio-diesel plant were hurt.

Two have been released from the hospital, the most seriously injured, with third degree burns, remains hospitalized in Toledo.

Fire investigators and OSHA reps were on scene mid-day. At that time, they were still trying to nail down the cause.

Mark Marentette/Defiance Fire Chief: " You had to have something that vaporized and was touched off by a spark, so we have to find out what that explosive vapor was."

NEUMEYER: " The plant located in Defiance in 2005, and it's very near a residential area. There are dozens of homes within just a couple of blocks of the operation."

Joe Trevino/Eyewitness: " I mean, it was loud."

Joe Trevino, 30 yards away, had pictures jarred off his living room wall.

Joe Trevino: " I knew there was going to be danger, you know, and I'm even talking to my wife about maybe selling this place and moving out of here."

Jeff Leonard/Defiance City Administrator: " The proper permitting was done. The necessary zoning was in compliance."

Bio-diesel plants are still a relatively new thing across the country.

The American Ag Fuels plant has left an impression on its neighbors that won't soon be forgotten.