But regulators said contamination and the potential for a further chemical-related disaster appear to be minimal.

The state fire marshal's office, a hazardous materials team and federal investigators from the Chemical Safety Board will conduct separate investigations in an attempt to find out what caused the explosion at the Water Street plant, which housed two businesses, CAI Inc. and Arnel Inc.

CAI Inc., a family-owned company, also manufactures inks at its Georgetown facility on Martel Way, in an industrial park next to Interstate 95. The company is run by President Vincent Sartorelli of Amesbury.

The blast damaged 90 homes - 25 beyond repair - and left as many as 200 people homeless.

Danvers Deputy fire Chief Kevin Farrell said hot spots and concerns about the structural integrity of the site prevented investigators from going in any sooner. Local police have been guarding the blast site and its surroundings.

Investigators from the Chemical Safety Board, a federal agency, will also arrive today to conduct their own assessment of the scene to determine whether they will conduct a full investigation.

The agency does not have any enforcement authority of its own but will make safety recommendations to businesses, labor unions and regulatory agencies, including the Environmental Protection Agency and the Occupational Safety and Health Administration.

That initial assessment will include interviews, reviewing documents concerning the chemicals kept at the plant and the processes that are used there, and an inspection of the site and the neighborhood.

Air samples taken at the scene showed no dangerous levels of any chemicals in the air, Mike Nalipinski, the EPA's on-scene coordinator, said. However, there was some contamination to the nearby Waters River from fire water runoff. A low-grade solvent used in the plant was found in the river.

On whether any serious chemical damage was done, Nalipinski said simply, "The jury's still out."

Ed Coletta of the state Department of Environmental Protection said that four air samples taken downwind of the scene found no detectable levels of chemicals used in the facility. The DEP checked for a list of nine substances used in the plant.

The DEP also tested six samples taken from the Waters, Porter and Danvers rivers and found measurable amounts of just one substance, methyl ethyl ketone, at amounts that were just a very small fraction of what officials believe would cause any damage. The highest level detected was 125 parts per billion in the Waters River, nearest the blast. By contrast, officials would be concerned by a level of 200,000 parts per billion, Coletta said.

Berms were set up around the scene Wednesday to prevent further runoff from yesterday's rains from entering the river, Coletta said, and Clean Harbors, an environmental remediation company, set up trucks equipped with vacuum pumps to remove any contaminated water from the scene. Storm drains were also plugged to prevent runoff from entering the river.

There are still hazardous materials on the site, including 6,000 gallons of chemicals stored in underground tanks, which Farrell said had not been compromised.

However, officials are concerned about the structural integrity of a number of steel 55-gallon drums used to store chemicals in the plant. There is concern that those metal drums may have been affected by the heat of the fire. As a result, a hazmat response team will move the drums into larger plastic containers.

Investigators have also inspected the storage facilities at Eastern Propane, just across the street from the blast site, and have determined that everything there appears to be stable.