The World Health Organization says it appears that the chemical "bromide" is the cause of a mystery illness that has affected nearly 400 people in Angola.

A WHO statement issued Monday says the agency found "extremely high levels of bromide" in blood plasma sampled from several patients affected with the illness. The agency has not speculated on how the victims were exposed to bromide.

Experts investigating the illness had suspected a toxicological cause, and they conducted tests for elevated exposure to lead, mercury and other chemicals.

The WHO says symptoms of the mystery illness include extreme drowsiness and difficulty walking. It says the outbreak is centered in the northern Angolan town of Cacuaco, where the first cases were reported in early October.

The tests confirming the presence of bromide were conducted in Germany. The WHO says additional samples are on their way to Britain for follow-up tests.