The fire at a lithium battery plant in South Korea that killed 23 workers in June broke out after the factory’s operator rushed production, ignored signs of danger and provided no safety training to employees, investigators said on Friday.

The fire in Hwaseong, south of Seoul, was one of the country’s worst industrial disasters in recent years and claimed the lives of mostly migrant workers. Nine people were injured. The company, Aricell, was rushing to fulfill a contract with South Korea’s military at the time of the fire, a police official said at a news conference on Friday.

The police said they had opened investigations into 18 people on charges including involuntary manslaughter, and filed for arrest warrants for two officials at Aricell. The Ministry of Employment and Labor said it had applied for arrest warrants for three executives, one of whom the police are also seeking an arrest warrant for, at Aricell and a related company for labor violations.

A person who picked up the phone at the Aricell factory in Hwaseong on Friday afternoon declined to comment. In the immediate aftermath of the fire, the company’s head, Park Sun-gwan, apologized for the disaster but said the factory had adequate safety measures and training.

The plant massively scaled up production after a batch of its batteries failed a quality test for the military in April, two months before the fire, according to the police. The company had to remanufacture the allotment for April in addition to filling an order that was due in June, the police official, Kim Jong-min, said at the news conference.

To meet the new production schedule, plant operators assigned untrained temporary hires en masse to the production lines for lithium batteries, which are known to be a fire risk, the police said. The company ignored problems that sprang up from the rapid production, including defective batteries, according to the police.