Georgia Department of Labor inspectors have discovered that the tank's pressure gauge read zero at the time of the blast that killed operators Heinrich ``Heins'' Kohl, 25, George Sanders, 42, and John Rowland, 35, early March 13.

``It had pressure in it even though the gauge read zero,'' said Jerry Hancock, the Labor Department's boiler pressure safety supervisor for the safety engineering division in Atlanta. ``What kind and where it came from is what we're trying to find out.''

The indicator's reading is among the latest findings in the labor department's ongoing investigation into a pair of explosions that also ignited a fire at the industrial plant off south Augusta's Tobacco Road. An internal team from BP Amoco, the federal Occupational Safety and Health Administration and the U.S. Chemical Safety and Hazard Investigation Board are involved in the probe.

The chemical safety board announced Wednesday it has sent a second investigative crew to determine if a hazardous chemical reaction could have been a factor.

The 9-foot-tall process vessel is used to collect byproducts in the production of Amodel, a heat-resistant plastic used in car parts. Just before the disaster, production had been shut down after a failed start-up attempt, according to a news release from the chemical safety board. Preliminary evidence suggests the tank contained process materials that continued to react together, generating pressurized steam, the board said.

As unsuspecting workers removed bolts from a 5-foot cover plate, the cover blew off and was sent flying, according to the release. The rupture also shifted the hefty vessel and bent 6-inch pipes.

The tank normally operates at 108 pounds of pressure. It is supposed to be completely depressurized before being opened.

``Our immediate priority is to understand the source of the pressure in the vessel and determine why workers had no warning of the imminent danger,'' chemical safety board member Dr. Andrea Kidd Taylor said in the release.

Mr. Hancock said Labor Department Inspector Carl Spitzer, who is assigned to conduct local investigations, will examine the gauge and the line connecting it to the tank.

Plant spokeswoman Pam Barbara said the company cannot comment on preliminary findings.

``I think we don't want to speculate until we know for sure,'' she said.

Reach Johnny Edwards at (706) 823-3225.