China said on Thursday it will send a team to Pakistan to help investigate a blast on a bus that killed 13 people, including nine Chinese workers, after it backed away from an earlier assertion that the explosion was a bomb attack, Reuters reported.

Zhao Lijian, a Chinese foreign ministry spokesman, told a regular briefing that China would cooperate closely with Pakistan in the investigation.

Wednesday’s blast sent the bus crashing into a ravine in Khyber-Paktunkhwa, where Chinese engineers have for several years been working on projects as part of Beijing’s massive Belt and Road Initiative.

On Wednesday, Zhao had called the blast a ‘bomb attack’ but Pakistan said a mechanical failure caused a gas leak that led to the explosion.

Senior Chinese diplomat Wang Yi met Foreign Minister Shah Mahmood Qureshi and urged Pakistan to investigate the blast but he stopped short of calling it an attack, according to a post on Thursday on the Chinese foreign ministry’s website.

But Wang told Qureshi that if it was indeed a ‘terrorist attack’, Pakistan should immediately arrest the culprits and punish them severely.

Wang, who is China’s State Councillor and foreign minister, said “lessons should be learned” and both sides should strengthen security measures for China-Pakistan cooperation projects to ensure their safe and smooth operation.