Though the police said they do not suspect foul play, the deaths have cast a shadow over one of Europe's leading electric battery producers, with Japanese car giant Toyota pulling its workers from the plant in response.

Three people at Swedish manufacturer Northvolt's "gigafactory" died in the space of six months, Swedish media reported.

Each died after working at the Northvolt Ett plant in the northern town of Skelleftea. Some 3,500 people work at the plant, Northvolt says on its website.

A 33-year-old man who worked as a cleaner at the factory was found dead in his bed on January 5.

On February 22 a 19-year-old man was found a day after his shift at the factory, having also died in his sleep.

After inital investigations, police concluded the deaths were not linked to each other or to their workplace.

A third employee, aged 59, was found dead at his home on June 5, media said.

Forensic examinations did not establish why any of the three employees had died, police said in a statement Thursday.

Further investigations would take into account the fact "these people worked at the same workplace", it added.

It said the third person "may have been exposed to a substance" and police "will be looking into this more closely".

As a result of the deaths, Toyota -- the world's largest car manufacturer -- suspended the work of its service technicians at the factory, it was cited as saying by specialist magazine Dagens Arbete.

Northvolt said the three deaths in six months were a "tragic coincidence".

It told AFP its internal investigation found that the employees were not exposed to toxic substances and had not suffered any accidents.

Deputy employment minister Paulina Brandberg said it was "very important to get clarity on what happened".

"It's terrible that three people have died after their work," she told Sweden's TT news Agency.

Northvolt is a cornerstone of European attempts to catch up with China and the United States in the production of battery cells, a crucial component of lower-emission cars.

Europe has been racing to produce more electric vehicles as part of its green transition, as the clock ticks down on an EU deadline to phase out the sale of fossil fuel-burning cars by 2035.

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