At least 38 people - including women and children - have been killed after unidentified gunmen opened fire on a convoy of 200 passenger vehicles traveling through a remote area of Pakistan.
The vehicles were attacked as they travelled through the tribal district of Kurram in Pakistan, close to the Afghan border, according to the area's deputy police commissioner.
The gunmen initially targeted the convoy's police escort, the provincial spokesman said in a statement.
Police were protecting the convoy following months of sectarian violence in the area, which has claimed dozens of lives this year.
Nadeem Aslam Chaudhry, the chief secretary of the Khyber Pakhtunkhwa province, told Reuters news agency Thursday's attack was "a major tragedy", with the death toll "likely to rise". At least 11 people were injured, he said.
Saeeda Bano - who was in the middle of the convoy - described to BBC Urdu how she feared she would be killed as she hid under the car seats with her children during the attack.
When the gunfire finally stopped after several minutes, she saw injured people and bodies lying in the road.
Details of exactly what happened are still emerging, but Javed ullah Mehsud, a senior administration official, told AFP "approximately 10 attackers" were involved, "firing indiscriminately from both sides of the road".
Women and children had hidden in nearby houses, while police hunted for the attackers, he added.
Most the passengers travelling in the convoy through the mountainous area were Shia, he said in an earlier statement.
Sunni and Shiite Muslim tribes have clashed repeatedly this year. An earlier series of attacks ended after a tribal council called for a ceasefire, according to Reuters news agency.
Then last month, there was another attack on passenger vehicles along a road in the region which killed 15 people.
The road Thursday's convoy was travelling along had only reopened in recent days, with travel limited to convoys with police protection.
Sectarian violence is often linked to land disputes in the region.
However, Kurram, in Pakistan's north-west, also borders several Afghan provinces which are home to anti-Shia militant groups, including the Islamic State group and the Tehrik-e-Taliban Pakistan (TTP).