"The newly-appointed security head of Bakwa was also among the dead," Khair Mohammad Baryali, a senior official at the office of the province's security chief, told Reuters news agency.
He said one policeman was wounded in the attack.
The previous police chief and Bakwa district governor were detained after the district was overrun by suspected Taliban fighters for less than 24 hours last month.
The government said it was investigating them for links to the Taliban.
Yousuf Ahmadi, a Taliban spokesman, told AFP news agency that "mujahidin" with the movement were responsible for the explosion.
Police post attacked
In a separate incident in Farah, two insurgents were killed and another wounded in a gunfight that erupted after they attacked a police post in Jowain district, police said. There were no police casualties.
Afghan officials blamed the attack on "enemies of Afghanistan," a term often used by Afghan authorities to refer to Taliban fighters and their allies.
Last year saw the worst violence in Afghanistan since the Taliban were forced from power by the US-led forces in late 2001.
More than 4,000 people died in fighting in 2006, including about 1,000 civilians. Suicide bombings jumped to 139 from 21.
Heavy fighting is expected in 2007 as the Taliban have warned that they have prepared thousands of suicide bombers.
Nato and Afghan troops last week launched their biggest operation so far to pre-empt a spring offensive by the movement.