GUMI, South Korea, Oct. 9 (Yonhap) -- Police said Tuesday that workers not properly following safety guidelines caused the gas leak at a chemical factory last month that killed five employees and sickened thousands of nearby villagers.

On Sept. 27, some eight tons of hydrofluoric acid, an acute poison that can damage lungs and bones and affect the nervous system, leaked from a chemical factory in Gumi, some 200 kilometers southeast of Seoul, according to police.

Unveiling closed-circuit footage, police said that the workers who died in the accident at chemical maker Hube Globe were presumed to have not been abiding by safety regulations nor wearing protective working clothes.

The gas leak occurred while several workers were attempting to unload hydrofluoric acid on the compound of the chemical plant.

Police said that they will further investigate officials at the plant and plan to book some of them on charges of negligence in causing the deaths of the workers.

Officials said more than 4,000 people have received medical treatment and hundreds of villagers in the affected region have been evacuated to a temporary shelter.

The government on Monday designated the gas leak area as a special disaster zone, which means it will be entitled to receive financial aid from the central government to cover rehabilitation costs.