Emergency workers and construction crews are sweeping through the Hungarian town of Kolontar hardest hit by a flood of toxic sludge, trying to clear roads and homes of the deep red mud and caustic water.

Kolontar mayor Karoly Tily said on Wednesday that he could not give a “reassuring answer” to residents who fear a repeat of Monday’s calamity, when 1 million cubic meters (35.3 million cubic feet) of toxic waste broke through a plant’s reservoir and engulfed several towns.

In Kolontar, a military construction crew is trying to assemble a pontoon bridge across a toxic stream so residents can briefly return to their homes and rescue belongings.

Hungary has declared a state of emergency due to the flood. At least four people have died, three are missing and 120 have been injured.