HOUSTON (AP) — Authorities say they've started a controlled burn of the remaining trailers of highly unstable compounds at a Houston-area chemical plant that flooded because of Harvey.

The Harris County Fire Marshal's Office said in a statement Sunday afternoon that the decision was made to take "proactive measures" to ignite the remaining trailers at the Arkema plant in Crosby through controlled means. The office said that the measures don't pose any additional risk to the community. The 1.5-mile evacuation zone around the Crosby plant is still in place.

Six of the trailers had remained after three others ignited in recent days, sending thick black smoke and tall flames into the air.

Arkema had said Harvey's floodwaters engulfed its backup generators at the plant, knocking out the refrigeration necessary to keep the organic peroxides, used in such products as plastics and paints, from degrading and catching fire.