LAWRENCE — A fire described as "too bright to look at" broke out in packages of chemicals at the train yard last night — the same chemicals cleared off the railroad tracks two nights ago after it spilled out of a derailed and punctured tanker.

Fire officials first worried the toxic smoke from the burning boxes of sodium chlorate might threaten residents of the neighborhood near the railroad crossing at Andover and Blanchard streets.

But a potential chemical scare ended without an evacuation similar to the one Thursday night when people were forced to leave a dozen houses on State and Andover streets for more than four hours.

"We just told people to stay inside their homes and keep their windows closed," Lawrence police Chief John Romero said.

ENPRO Environmental Services of Newburport, the company that conducted the decontamination and cleanup efforts after Thursday's chemical spill, were back late last night investigating what caused the fire and making sure the public was protected from any further environmental or health threats.

Sodium chlorate is used to produce chlorine dioxide for bleaching paper. In large amounts, it can become volatile if it comes in contact with water or other organic items such as wood or other natural debris, said Lawrence fire Chief Peter Takvorian.

"If any wood or paper got into the boxes with the product, it could certainly cause the chemical to generate some heat. And that may have been the reason why it ignited," Takvorian said last night.

Takvorian said an official cause of the fire is still to be determined.

The chemical that caught fire last night was the same taken from about 900 feet of railroad track Thursday after the tanker derailed. Described as white and granular like sugar, the cleaned-up chemical was put in cardboard boxes lined with plastic. The remainder of the chemical contained in the damaged tanker was also being stored at the site but in nearby Dumpsters, Takvorian said.

Takvorian said fire officials were concerned about putting out the blaze to keep a dangerous cloud of smoke from threatening the neighborhood. The quantity of the chemical was small enough that extinguishing the fire didn't pose a serious threat, he said.

"This is a small amount. So, we can definitely dilute it with water and keep the smoke down," the fire chief said.

Takvorian said the runoff water from last night's fire would be tested to determine whether there's a potential health or environmental threat. State environmental and hazardous waste officials were at the scene last night.

Meanwhile, public safety officials were taking precautions to restrict people from coming in contact with any smoke from the fire. Auxiliary police Chief Jay Jackson and his volunteers were involved in crowd and traffic control, blocking off Andover Street from South Broadway to Parker Street. They also blocked off Winthrop Avenue between Andover and Salem streets.

"All 22 officers were called in tonight just in case we had to evacuate the homes. We took special precautions tonight," Jackson said.

Last night's incident disrupted rail service through the area. Officials said one Amtrak Down East train to Portland was canceled. In another one, 90 passengers were transferred to buses in Andover to take them to Portland.

The commuter rail service to Haverhill needed buses to take passengers from Andover to the Lawrence train station, avoiding the rail yard. The fire broke out at about 9:15 and was extinguished in about an hour.

It was a fire that drew attention quickly because of flames that some observers said they couldn't bare to look at

"When the fire was really going, it was a white bright light — too bright to look at. It was burning like phosphorus," Sgt. John Nicoletti said.

The immediate concern for police and fire officials was the smoke

A fire involving sodium chlorate may produce irritating, corrosive or toxic gas. Police officials were initially aggressive in crowd control, ordering people to stay behind the yellow crime scene tape or risk arrest.

Another concern for fire officials is the runoff from the fire, which can cause pollution problems. Officials were monitoring that situation early today.

Once the fire was out and the smoke had dissipated, police allowed the media and spectators to move a little closer to the railroad crossing, but still maintain a distance. Smaller flames reignited. Fire officials decided to let those burn themselves out.

Firefighters remained at the scene overnight as ENPRO Environmental Services work crews continued their cleanup efforts.

Shortly before 1:30 a.m., fire officials had given clearance for rail service to resume in the area.