Authorities said on Friday that three others were being sought for the deadliest attack on foreign civilians since the fall of the Taliban.
They have been facing growing problems in the north after Thursday's slaughter of Chinese contractors working on a flagship road project.
Assailants attacked the workers' tents in a desert camp 250 kilometres north of Kabul and shot them as they slept. An Afghan guard was also killed.
General Muhammad Daoud, the military commander in Kunduz province, said two suspects were picked up on Friday in connection with the killing and three more were being sought.
He gave no details and said it was too early to say whether anti-government fighters, Taliban, al-Qaida or followers of Afghan commander Gulbuddin Hekmatyar, were responsible.
Authorities are not certain if
Gulbuddin Hekmatyar is at fault
Earlier on Friday morning, Afghan and German military planes brought the dead and five wounded from Kunduz to Kabul airport.
Further south, fighters launched a barrage of rockets at a heavily guarded convoy of UN and Afghan officials in a stronghold of Taliban-led loyalists, but caused no injuries.
More than 30 vehicles on Friday were picking their way through the mountains of Gomal, a lawless district in Paktika province, 230 kilometres south of the capital, Kabul.
"Suddenly they were attacked with rocket-propelled grenades fired from a long distance," provincial police chief General Muhammad Rahim Alikhil told The Associated Press.
Some of the dozens of Afghan and American troops guarding the convoy returned fire, and the assailants fled, Alikhil said.
The vehicles were taking Paktika Governor Ghulab Mangal, UN officials, and American military officers on a mission to prepare for government administrators to return to the province, which is considered a stronghold of the ousted Taliban government.
Several journalists, including a contingent from ABC News, were accompanying the convoy.
Poor security has prevented the UN from registering voters in much of the south and east, hampering preparations for national elections slated for September.