ABUJA — At least 37 people died in Nigeria's oil-producing Rivers state after inhaling toxic fumes while siphoning crude from a loading vessel Thursday.

The Youths and Environmental Advocacy Center reported the deaths Friday, saying victims were funneling crude into boats from an illegal tapping point. Many people remain missing, the group said, citing information gathered through its network of local volunteers.

Rivers State police spokesperson Blessing Agabe confirmed the fatalities Friday and said the victims were allegedly attempting to steal crude oil. Police did not provide an official death toll but said an investigation into the circumstances is underway.

Oil theft is endemic in Nigeria's Niger Delta region. Decades of exploration there have left local populations in poverty and caused significant environmental damage. The Nigerian government has said oil theft costs the country several billion dollars annually and has stepped up efforts to combat the illicit practice.