Friday's attack was the latest in a wave of bomb attacks that have killed and wounded dozens of people in Afghanistan in the past few months.
Shadi Khan, the former chief of Deshu district police in the southern Helmand province, was killed and two of his bodyguards were wounded when his car was hit by the blast, Haji Mohammad Wali, spokesman for the provincial governor, said.
He blamed the Taliban but drug running and tribal and factional rivalry are known to have caused violence in the province in the past.
Amanullah Jan, intelligence chief in the Helmand province, said the incident occurred in Shajoi district on Friday, 180km west of Kandahar.
"We believe it is the work of Taliban," Jan said, although he did not offer any evidence.
Separately, Taliban fighters searched a passenger bus after stopping it in a remote area of southwestern Oruzgan province on Thursday, and took away an Afghan soldier after finding an identity card in his pocket.
The bullet-riddled body of this soldier was later found on a road, Jan Mohammed Khan, the governor of Oruzgan, said.
He said they dispatched soldiers to the area to trace and arrest the Taliban fighters after getting information about the killing, but "so far we have made no arrests".
US personnel killed
Two US service personnel were killed in a roadside bomb blast in southeast Afghanistan on Friday.
The two, described as US service members, were in a convoy in Paktika province, near the Pakistani border, when their vehicle was hit by the blast.
A US soldier and an Afghan interpreter were wounded in the attack, the US military said in a statement.
The Taliban has claimed responsibility for the assassination of Mawlavi Abdullah Fayaz, a prominent critic of the Taliban who was killed on Sunday by armed men on a motorcycle.
But the group deny attacking a Kandahar mosque on Wednesday in which 20 people died. A Taliban spokesman condemned attacks on places of worship.
The Taliban, whose government was ousted as a result of US-led attacks in late 2001, have recently stepped up their attacks against Afghan and and UN-backed multinational forces, undermining assertions from Afghan officials that security has improved in Afghanistan significantly.