The dead include employees, contractors and suppliers to the plant, said Carlos Solorio, secretary general of the local branch of the national mining and metallurgical workers’ union.

“Everyone was working for the company,” Solorio told Reuters. Nine of the deceased were union members, he added.

ArcelorMittal, the world’s largest steel producer, was not immediately available for comment on the report of the deaths, which were initially made public in a video posted on social media by the union’s president, Senator Napoleon Gomez Urrutia.

Solorio said the 21 deaths had occurred since April, and that the workers likely contracted the virus outside the plant, given the strict sanitary protocols adopted by the firm. The plant employs more than 7,000 people, according to the union.

ArcelorMittal has said it has made substantial donations to help tackle the virus in Lazaro Cardenas, a major port on the Pacific coast.

With over 385,000 confirmed cases and nearly 43,500 deaths as of Saturday, Mexico has the world’s fourth-highest death toll from coronavirus. Lazaro Cardenas has registered 2,651 cases and 139 deaths to date, official data showed.