The killing of women and children at a checkpoint near the southern city of Najaf involved the same unit that suffered a suicide attack in which four American servicemen died.
It underlines just how jittery the troops are on this key highway between the south and the north.
If vehicles approach us on the road we may open fire. We need this road open and we can't delay for vehicles that might approach us and might contain suicide bombers
They are edgy about any car that approaches them because of this fear of suicide bombings.
I think they are working on the basis that if vehicles do not stop immediately they are regarded as hostile.
One commander told me: "If vehicles approach us on the road we may open fire. We need this road open and we can't delay for vehicles that might approach us and might contain suicide bombers."
Still, they regard the incident that happened on Monday as a tragic mistake.
Troops are wary of all Iraqis approaching them
They say they will do everything they can to avoid it happening again.
But on the other hand, if people do not stop at checkpoints, there might be a reoccurrence.
There is a war going on and the soldiers are braced for suicide attacks.
It is very difficult for the forces here, and they realise what the consequences are if they get it wrong.
They are at a huge risk of alienating the local population.
It makes it impossible for the Americans to develop any kind of relationship with the Iraqi people.
It could be that is what the suicide bombers had in mind.
Normal encounters become much more difficult, with the Americans being cool until they are certain any vehicle that approaches them does not carry weapons or explosives.
A suicide attack killed four US soldiers near Najaf
It is very difficult for the troops to train for this, particularly in a fluid situation.
If it is a fixed situation they can have barbed wire and chicanes, and get people to slow down so they can approach the vehicle in a way that helps them determine whether they are friendly or not.
But they have just taken over a highway and are moving things up that highway, and those kinds of systems are not in place.
That is not to say they will not be in place, but we are at the early stage of a war.
In the meantime, soldiers who are being harassed will be very cautious and tense when they see Iraqis approach them.