At 8 p.m. on Monday, about 20 workers of the Golden Victory Vietnam factory in the northern province were rushed to a local hospital after feeling heaviness in their chest and having problems breathing.

Between Tuesday and Thursday, the provincial hospitals took in more workers from the same factory with similar symptoms, with the total number of patients going up to 125.

At first some workers had difficulty breathing, heaviness in the chess, and felt faint, and then others felt it too, but this could have been psychosomatic, said Ngo Gia Tu, Vice Chairman of Nam Dinh People's Committee.

Tu said the province has asked the Ministry of Public Security's Criminal Science Institute and the provincial investigation police to work together and find out what happened.

For now, factory has suspended operations. At the time of closing, the Golden Victory factory had about 7,000 workers.

As of Thursday afternoon, three workers, one of them pregnant, were in critical condition. The condition of others had improved.

The factory in Nghia Hung District is one of many overseas manufacturing facilities owned by Hong Kong's Stella International Holdings Limited which manufactures footwear products. Stella lists Guess, Timberland, Michael Kors, Prada and Marc Jacobs among its customers.

The company signed an exclusive agreement with the Golden Star Company Limited to manufacture men's casual and fashion footwear in Vietnam in 1998, according to its website.

Apart from Vietnam, its mother company Stella also runs factories in several provinces in China and Indonesia.

In 2015, about 5,000 workers at its factory in southern China went on a strike demanding better benefits, including housing assistance, according to Reuters.