A delegation from Kabul headed to the province to investigate the attack, with a focus on the motive and whether there is a connection to local police commander Bilal Shah, a provincial official said by phone on the condition of anonymity because of the sensitivity of the situation.
The protest rally involved hundreds of people, and the bomber detonated a suicide vest amid the crowd, which had blocked the main highway that links Kabul with Pakistan.
The protesters demanded the dismissal and arrest of Shah, who has been accused of severe abuses, including torture, abductions and killings.
Shah was briefly arrested recently because of allegations from residents but was later released.
“He was free when the blast took place, but has been jailed since. He will be referred to judicial organs, and it has become a serious issue that is being looked at by the delegation from Kabul,” the official said.
Relatives of victims of the Tuesday attack have continued blocking the highway, according to witnesses.
The attack occurred amid a surge in strikes by the Islamic State and Taliban insurgents elsewhere in the country. More than 200 people have been killed in attacks since last week.