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© Brynn Anderson / Omaha World-Herald
The roof caved in at the International Nutrition plant Monday in Omaha, Nebraska.
Four people were killed and 18 others were injured, at least four of them critically, in two separate explosions at industrial plants in Nebraska and Oklahoma on Monday, authorities said.

Two died when the three-story International Nutrition Inc. animal feed plant collapsed and burst into flames about 10 a.m. (11 a.m. ET) in Omaha, Neb., officials said. Four of the 17 other people injured there were critically wounded, interim Omaha Fire Chief Bernie Kanger said Monday night.

Witnesses described hearing a loud noise and then seeing a fireball seconds before the building collapsed.

Six hours later, two more people died and a third suffered head burns when a furnace exploded at the Mid-America Steel and Wire facility in rural Madill, Okla., the Marshall County Sheriff's Office told NBC News. The condition of the third person couldn't immediately be determined Monday night.

Both of the victims of the Omaha incident were on the second floor of the International Nutrition building. One of the bodies was recovered late Monday afternoon, but the second remained at the scene Monday night. Operations to recover that body were suspended Monday night because of severe weather.

All of the 36 other employees who were in the building when it collapsed were accounted for, Kanger said. Ten were taken to hospitals, four of them with critical injuries. Seven other people with minor injuries were treated at the scene.

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A view from the rear of the animal feed plant.
Investigators said the cause of the collapse remained unknown. The recovery and investigation were moving slowly because the remaining structure was highly unstable, they said.

"There was a third-floor collapse and a second-floor collapse onto the first floor, so there is substantial damage," Kanger said. "We've got tens of thousands of pounds of concrete, reinforced concrete and steel" to deal with.

Kari Cook told NBC station WOWT that her fiancé, John Broderick, sent her a text message at 10:09 a.m.:
"Major accident. I'm hurt and trapped. I love you."
Broderick, who works on the second floor of the building as a supervisor, was rescued and taken to the hospital with broken ribs and a deflated lung, Cook said.

Few details were available in the explosion in Madill, in southern Oklahoma about 100 miles southeast of Oklahoma City.

A Marshall County emergency dispatcher said the agency received a call that a furnace had exploded. The scene was blocked off and had been cleared by 10 p.m. ET

The Omaha company has been fined at least twice before for safety violations, according to records of the Occupational Safety and Health Administration, which will be in charge of the investigation. The first citation came in August 2002, after a rotating part killed an employee who fell in a mixing tank. The second came in November 2011 after a safety inspection identified six "serious" violations.

No violations were on record for the Oklahoma steel plant.