The Taliban is claiming responsibility but that is probably propaganda. What we know is that two more Americans were shot by an Afghan in uniform - possibly a policeman - in what should have been the safest building in Afghanistan; the Interior Ministry.
Afghan security officials said those killed were an American colonel and major. Local media reports suggest the incident followed a "verbal clash".
Nato commander Gen John Allen said all Nato personnel were being recalled from Afghan ministries on security grounds.
A UK embassy spokesperson had earlier said all British civilians were being withdrawn from the ministries in what was hoped would be a temporary measure.
The shootings come amid five days of deadly protests over the burning of copies of the Koran by US soldiers.
Gen Allen said he condemned the attack, adding: "We will pursue all leads to find the person responsible. The perpetrator of this attack is a coward whose actions will not go unanswered."
He added: "For obvious force protection reasons, I have also taken immediate measures to recall all other Isaf personnel working in ministries in and around Kabul."
Isaf spokesman Brig Gen Carsten Jacobson told the BBC that Nato could not yet confirm the identity of those killed.
He would also not be drawn on any link to the Koran protests.
Gen Jacobson said: "We have seen an emotional week, we have seen a busy week - but it would be too early to say this incident was linked to what we have seen over the last days.
"At the moment, it is too early to say what actually happened."
Early reports suggest the two officers were shot in the ministry's command and control centre.
The BBC's Bilal Sarwary in Kabul says this is where representatives of 34 provinces meet to plan security.
He quotes sources as saying that Interior Minister Bismullah Khan was having a meeting with senior Western officials elsewhere in the building when the shooting took place.
Pretty soon, all Afghan security personnel will have to be disarmed if they are going to interact with Americans. You can be sure that for the present, meetings between Americans and Afghans are going to be tense affairs.
Still waiting for President Karzai to apologize for the murderous behavior of his troops.