Nearly 400 Angolans fell ill over the past several weeks and health officials may have discovered the reason: bromide poisoning, according to the World Health Organization.

Health officials were at a loss when a mysterious outbreak hit Angola, with nearly 400 people becoming ill. The first case of the disease was reported in October in the capital, Luanda. More than half the patients are under 15. At least four people have died.

The World Health Organization said Thursday that initial tests performed in Munich, Germany found “extremely high levels of bromide” in the blood samples analyzed. Additional samples are currently on the way to a laboratory in the UK for testing for bromides, the WHO said.

“We are still in the preliminary stages of investigating this outbreak and have a lot of unanswered questions,” said Peter Ben Embarek, a WHO food safety expert, as quoted by the Associated Press.

“The problem is that sodium bromide doesn't smell of anything and it looks like salt, so it would be very easy to confuse it with something else,” Embarek said.

Officials do not know yet how people were exposed to bromide. The AP notes that the chemical is commonly used in Angola's oil industry. Bromide can be found naturally, for example in seawater, but in other forms, it can be toxic or even fatal.

Angolans that fell ill presented symptoms such as extreme drowsiness and inability to walk unaided, which led WHO and Angolan health experts to suspect the cause is toxicological, per the AP.

The agency is sending more doctors and experts in food safety and laboratory work to Angola, to treat people and investigate the outbreak.

Additional cases continue to appear, with seven more people reported sick on Thursday. The ill are being treated at the Municipal Hospital in Cacuaco, the health agency said.