LAWRENCE — Firefighters yesterday afternoon returned to the scene of Saturday night's chemical fire in a train yard behind Andover Street, but this time they went to douse some smoldering sections of the track.

The fire did not reignite yesterday, but several spots seemed to be affected by heat from direct sunlight and showed the potential for another outbreak, Deputy fire Chief John Marsh said.

Fear of toxic smoke forced residents of State and Andover streets to evacuate their homes for more than four hours Thursday night after a tanker derailed and spilled sodium chlorate along 900 feet of track.

Most of the spilled powder was cleaned up and stored in cardboard boxes lined with plastic, but the chemical caught fire Saturday night and took firefighters about an hour to get under control. Some of the sodium chlorate is still on the track because it's difficult to remove all of the chemical, Marsh said.

"I hope this is the end of it," he added.

Sodium chlorate is a white powder that can become volatile if it reacts with water, wood or debris. It is used to make chlorine dioxide to bleach paper.

The company in charge of the cleanup and decontamination, ENPRO Environmental Services of Newburyport, also sent personnel back to the scene yesterday. No one from the company was available for comment yesterday.

The state Department of Environmental Protection spokesman also could not be reached.

Marsh went to monitor the situation about 1 p.m. and decided to send firefighters to the area, which is at the end of Easton and Farley streets near fenced-off car lots.

Firefighters took the fence apart and set up an aerial ladder so they did not have to go on the tracks — where a regular commuter train and a high-speed train might be coming — and could spray water, Marsh said.

Firefighters can shoot more water using an aerial ladder than by simply holding a line on the fire, he said. Although small amounts of water can react with sodium chlorate, causing a dangerous toxic cloud to form, Marsh said massive amounts of water are effective at fighting this type of chemical fire.

Between 1 p.m. and 3:45 p.m., firefighters applied "copious amounts of water" on the smoldering spots, he said.

Police Chief John Romero said Andover Street was reopened to traffic, but he referred specific questions about the cleanup to the Fire Department.