The reported American attack on a convoy thought to be carrying Saddam Hussein and/or his sons last week would have been legal only if there was substantial evidence indicating their presence, according to Amnesty International.
(Update to this report: on 24 June, the US Defence Secretary Donald Rumsfeld said that he had "no reason to believe" that anyone in the Iraqi leadership had been in the convoy. He did not elaborate on why five Syrian border guards were wounded in the incident)
But if there had just been a hope and there was the risk of killing innocent civilians instead, then it would not be justified, said Amnesty International's Legal Director Claudio Cordone.
Mr Cordone told BBC News Online: "If the convoy really involved Saddam Hussein or other military leaders, it would have been a legitimate target. The war has not ended. President Bush announced the end of 'major combat operations' only. The laws of war still apply.
"But the point is that you have to take all precautions. You are allowed to attack a military target even if civilians are there. But you have to use the principle of proportionality and weigh the value of the target against the risk to civilians. And you have to show that you checked that the target was a military one.
"If Saddam was known to be there, the target would probably have been legitimate, whoever else was. It is a difficult calculation. If you are in doubt, you are required to hold back under the precautionary principle."
This is the third attempt to strike Saddam from the air
Mr Cordone drew a distinction between this reported incident and others, in which lethal force might not be justified.
"There is a difference between this kind of incident and firing on crowds of demonstrators. In situations where you are already in control, you have to use methods of policing not of combat," he said.
The relevant article of international law is Protocol 1 Additional to the Geneva Convention 1977.
Article 51 says: "The civilian population shall not be the subject of attack."
Article 57 deals with the precautions which have to be taken. The attackers "should do everything feasible to verify that the objectives to be attacked are neither civilians nor civilian objects."
The United States has not ratified the additional protocol, but Mr Cordone said that international law regarded the protocol as binding in the custom and conduct of war.
US desperate and determined
The reported convoy attack shows that the United States is becoming increasingly desperate, and determined, in its search for Saddam Hussein.
It badly needs a success right now. American soldiers are being killed. No weapons of mass destruction have been found. Iraq is recovering only slowly.
Hence the risk the Americans were willing to take. The intelligence linking Saddam and Uday and Qusay Hussein to the convoy is thought to have been based on a monitored satellite telephone call.
Convincing the world that Saddam was dead would require describing in detail how those samples were acquired and how the matching DNA was examined
The possibility of killing totally innocent people must have been taken into account. The prize of getting Saddam Hussein or one of his once powerful sons, it seems, was worth the price others might pay.
A DNA investigation of human remains is said to be under way.
Need for transparency
The unit in charge of the search for Saddam, Task Force 20, is said to have DNA samples of a number of former Iraqi leaders. Establishing the identity of their bodies would not be difficult in that case.
But convincing the world that Saddam was dead would require describing in detail how those samples were acquired and how the matching DNA was examined.
The legacy of doubt left by the intelligence failure on the WMD issue would have to be overcome.
King Abdullah of Jordan compared the sightings of Saddam, who may or may not be dead, to those of someone who is: "It's like Elvis," he told ABC television. "You're getting so many stories, left, right and centre."
But Saddam unaccounted for is a threat.
Which is why Task Force 20 was set up.
It has two main elements. There are intelligence officials from the CIA and the National Security Agency which is the US government's electronic eavesdropping organisation.
Added to them are US special forces who can call on heavy firepower. The attack on the convoy is variously said to have been carried out by an unmanned Predator drone firing a Hellfire missile or a giant AC-130 gunship.
Whatever aircraft was used, this was at least the third specific attack aimed at Saddam Hussein from the air.
The first came on 20 March, just before the war started, when a farm was attacked in the hope that Saddam would be killed and war averted.
Then on 7 April, with Baghdad about to fall, a building where Saddam was thought to be eating was bombed.
The best break for Task Force 20 has been the recent arrest of one of Saddam's closest aides, his secretary and bodyguard, Abid Hamid Mahmoud al-Tikriti.
He has reportedly told his American interrogators that Saddam Hussein and his sons survived the war.
He has further said that he himself went to Syria with Uday and Qusay Hussein but that they had to return to Iraq after being expelled.
That would fit in with reports at the time that Syria was an intended place of refuge for the Iraqi leadership, despite traditional hostility between the two regimes.