At least half of a sprawling Slim Jim processing plant was damaged in a fatal explosion, and parts of the building may need to be removed before federal investigators can go inside, officials said Thursday.

A day after authorities pulled three bodies from the site, structural safety engineers with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives brought lasers and other tools to assess the building's stability. Those experts will determine which parts of the idled plant to shore up or take out before agents can pinpoint the source and cause of the blast.

"When the fatalities occurred at the time of the explosion, that may have been uncontrollable," said ATF spokesman Earl Woodham. "However, any damage or any injury to a person from this point on is purely controllable."

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The explosion Tuesday morning, with 300 people on duty, injured at least 38 people, and three firefighters were treated for inhaling ammonia fumes. Doctors are treating four people with critical burns covering between 40 percent and 60 percent of their bodies.

Woodham said at least half the 500,000-square-foot ConAgra Foods Inc. building has been damaged and that massive beams and very heavy concrete pillars are unstable.

Officials have declined to discuss the possible causes of the explosion. Woodham said he agents aren't ruling anything out.

"If we determine the cause we're also going to be able to determine if it was deliberate act, an act of nature, an industrial incident, or anything like that," Woodham said. "Everything's on the table and we are looking at everything."

ConAgra employee Harold Harris, 40, likened the explosion to a sudden Category 5 hurricane. He said the blast threw him against the wall, buried the doorway in rubble and sent part of the roof falling. At least three parts of the roof collapsed, and an exterior wall crushed several cars parked next to the building.

Dave Jackson, a spokesman for Omaha, Neb.-based ConAgra, has said someone called the plant over the weekend and threatened to start a fire, but authorities have said they knew of no link between the call and the blast.

The company has 25,000 employees worldwide and makes brands such as Chef Boyardee, Hunt's tomato sauce, ACT II popcorn and Hebrew National hot dogs.