GARNER, N.C. (AP) ó Federal agents are ready to begin investigating an explosion that killed three people and leveled part of a Slim Jim snack factory, but it may be a few days before they can safely enter.

Once the building is stabilized, a 25-person team of criminal investigators, forensic specialists and engineers will try to determine what caused the blast, Earl Woodham of the federal Bureau of Alcohol, Tobacco, Firearms and Explosives said Thursday.

The explosion ripped through the ConAgra Foods Inc. plant in Garner, just south of Raleigh, on Tuesday morning, collapsing walls and part of the roof. About 300 people were at work at the time, and dozens were injured, including four who were critically burned.

Officials identified those killed as Barbara McLean Spears, 43, of Dunn, and two Clayton residents: Rachel Mae Poston Pulley, 67, and Lewis Junior Watson, 33.

Woodham said search and rescue teams are the only people who have entered the plant so far. Some federal agents will begin interviewing workers while others will help clear debris.

“It has to be completely safe before we send anyone in there,” Woodham said. “There has been no one in the scene for investigative purposes at all.”

Woodham said the team will determine whether there was criminal activity and hand the case to local authorities if not.

He called the scene “overwhelming,” but told The Associated Press on Wednesday night that nothing about it suggested foul play.

ConAgra spokesman Dave Jackson has said someone called the plant over the weekend and threatened to start a fire, but authorities say they know of no link between the call and the blast.