TEKAMAH, Neb. — Officials attempted Tuesday to determine how anhydrous ammonia escaped from an underground pipeline in northeast Nebraska, killing a man who was driving by.

Magellan Midstream Partners, of Tulsa, Oklahoma, said in a news release that the leak occurred Monday night on its 8-inch pipeline system. The Burt County Sheriff’s Office said the odor was reported around 9:20 p.m. Monday, about eight miles north of Tekamah, which is nearly 40 miles from Omaha.

“We do not know the cause of the incident at his time,” said Magellan spokesman Bruce Heine.

Around 10:05 p.m., a 911 caller reported that a motorist needed help. Hazmat workers and Tekamah firefighters reached the man and moved him to a safe area, but he was pronounced dead at the scene, the sheriff’s office said. The sheriff’s office identified the man as 59-year-old Phillip Hennig of Tekamah, Nebraska.

“Our thoughts and prayers are with the family,” Heine said.

Anhydrous ammonia is a farm fertilizer with suffocating fumes that cause breathing difficulty and irritation to eyes, nose or throat.

The company said about 40 people who live in 23 homes near the pipeline leak still weren’t able to go home Tuesday afternoon while officials monitored the fumes.

Nebraska Emergency Management Agency spokeswoman Jodie Fawl said a state emergency response team was dispatched Tuesday morning, but she had no other information.

Magellan owns and operates the pipeline system, which originates in Texas and terminates in Minnesota, Heine said.