ISLAMABAD (AP) — Pakistan on Thursday said an initial investigation into a deadly bus crash the previous day in the country’s northwest found “traces of explosives” at the site, raising the possibility the incident was a terror attack.

A bus carrying dozens of construction workers fell into a ravine Wednesday in the district of Kohistan in Khyber Pakhtunkhwa province after what authorities said was a gas leak in the vehicle that caused an explosion. At least 13 people, including nine Chinese, were killed and at least 36 people were injured.

Chinese Embassy in Islamabad promptly said the bus was attacked, without elaborating.

No group has so far claimed responsibility for the deadly incident.

After the explosives find, Pakistani Information Minister Fawad Chaudhry said “terrorism cannot be ruled out.”

Prime Minister Imran Khan was “personally supervising all developments in this regard,” Chaudhry said, adding that Islamabad was in contact with the Chinese Embassy. He provided no further details.

Chinese engineers and construction workers are helping Pakistan build a dam in Kohistan. The bus with the workers was en route to the construction project when the explosion happened and the bus careened, plunging into a ravine.

Road accidents are common in Pakistan, where motorists largely disregard traffic rules and safety standards and drive on damaged roads, particularly in the mountainous terrain in the north.