Thursday's victims were driving on the main highway linking the national capital, Kabul, with the southern city of Kandahar when they were attacked in Zabul province's Shahjoy district, said Ali Khail, provincial government spokesman.
Initial reports suggested the victims had been working for an international aid group and that seven people had died, but Khail said only six were killed. He said he was waiting for confirmation of their identities.
They had been taking the body of the engineer to Kabul, said Naik Mohammed, a doctor at a hospital in the town of Qalat where their bodies were taken.
The engineer was killed on Wednesday along with four other Afghans who were working on a US-funded project to help end opium farming in the south of the country. Taliban fighters were also accused of being behind Wednesday's attack.
The two assaults were about 250km apart from each other and it was not immediately known whether they were linked.
Afghan police and soldiers have been deployed to the location of Thursday's killing, the official said.
Afghanistan's foreign minister said on Thursday that his government was in contact with the kidnapper of an Italian hostage, and he suggested she was safe.
Abdullah Abdullah's comments came as a deadline set by the purported kidnapper passed. The kidnapper had threatened to kill hostage Clementina Cantoni unless his demands were met.
Clementina Cantoni was working
on projects helping Afghan widows
"They are aware of the safety of the kidnapped," Abdullah told reporters in Tokyo, where he is on a three-day visit. "But I have no further information."
A man claiming to have kidnapped the 32-year-old relief worker in the Afghan capital threatened in a telephone interview broadcast on local television to kill her unless his demands were met by Wednesday evening.
Police played down the threat to Cantoni, saying that the ultimatum was probably not serious.
Cantoni has been in Afghanistan since 2002, and was working for Care International on a project helping Afghan widows and their families.
Her kidnapping was the latest in a series of attacks targeting foreigners in Kabul, reinforcing fears that fighters or criminals are copying tactics used in Iraq.
Fighters have stepped up attacks on military and civilian targets after a winter lull, further impeding aid to the impoverished region. Aid workers have been targeted as part of a drive to undermine recovery under the US-backed government that replaced the hardline militia.
US and Afghan troops have fought a series of bloody battles with fighters since early April. About 150 fighters have been reported killed in the period, along with 30 members of the Afghan security forces, three US troops and a Romanian soldier.