The U.S. military has launched a probe into a helicopter crash that left two American soldiers dead, officials said Tuesday.

An AH-64 Apache helicopter from the U.S. Army 2nd Infantry Division crashed onto a mountain road late Monday near Wonju, Gangwon Province, some 100 kilometers east of Seoul.

The accident took place 20 minutes after the attack helicopter departed from Osan Air Base in Pyeongtaek, Gyeonggi Province. The crew members were on their routine night low-fly training mission, the U.S. authorities said.

The bodies of the two soldiers were recovered from the helicopter, Wonju firefighters said. The crewmen’s identities have not been disclosed.

The investigators suspected that the attack helicopter hit a high-voltage line or a steel tower in the foggy weather. A disconnected line and upper part of a tower were found among the wreckage.

“I heard a ‘bang’ sound and saw flames here and there around the road along with the broken line,” a witness resident said, according to a police report.

The weather condition was appropriate for flying as the visibility was 8 kilometers with the wind speed at 2 meters per second, Korean military sources said.

No further causalities or property damages were reported. The accident site was 500 meters from residential property.

The U.S. military investigators collected the black box and the wreckage for closer investigation. The probe will likely take about a month, they said.

The traffic around the accident scene was restricted Tuesday.

The Apache helicopter, armed with a 30-millimeter M230 chain gun, Hydra 70 rocket pods and 16 hellfire missiles, is one of the most advanced attack helicopters in service. It has day or night or all-weather flight capabilities and can attack tanks or bunkers up to 8 kilometers away.

By Lee Hyun-jeong (rene@heraldcorp.com)