Today investigators are heading back to the scene to determine what caused a plant explosion in Omaha that has caused at least two fatalities and left almost a dozen injured. NBC's Gabe Gutierrez reports.

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.

The victim whose body was recovered Monday was identified by the Associated Press as Keith Everett, 53. The second victim, whose body was still trapped, was identified by the Omaha World-Herald newspaper as David Ball, 47.

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.

About 50 Omaha firefighters battled the blaze, Kanger told Reuters.

A view from back side of international nutrition. Thoughts and prayers to all involved. #OPD pic.twitter.com/kTQRgERuFl via @OPDofcBossman — Omaha Police Dept. (@omahapolice) January 20, 2014

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. The Occupational Safety and Health Administration will investigate the cause, but it could be weeks before they finish their investigation.

"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.

The blast knocked out the lights in the building, the AP said, sending workers to scurry to safety in pitch-black darkness.

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.

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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.

NBC News' Elizabeth Chuck contributed to this report.

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