The Associated Press

APEX, N.C. — The exact cause of a chemical fire that prompted evacuations will probably never be known because much of the evidence was destroyed in the blaze, federal investigators said Thursday.

One week after the fire at EQ Industrial Services in suburban Raleigh, officials from the U.S. Chemical Safety and Hazard Investigation Board said it will be difficult to piece together what happened.

"We might be able to come up with some likely scenarios, but that would be the best that we would hope for," said Robert Hall, who is leading the agency's investigation. "The evidence is extensively destroyed."

The fire started Oct. 5 with a series of explosions that left a yellowish haze over much of the town. Fearing the possibility of toxic air, town officials urged as many as 17,000 people to evacuate until a day and a half later, when tests showed the air was safe.

The chemical board, an independent federal agency that looks at industrial chemical accidents, has interviewed company employees, photographed and examined the site, and interviewed town officials and residents.

Hall said the team would return to Washington, D.C., on Thursday to start analyzing the material. The board is expected to decide within three weeks whether to launch a full investigation, which could take about a year.