After 9/11, President George W Bush made an apparently simple statement about Osama Bin Laden: "Wanted - Dead or Alive."

But the question whether to kill him or capture him was a subject of controversy in Washington for long periods during the 15-year hunt for the al-Qaeda leader.

For the five years before 9/11 the "dead or alive?" question was left unanswered, and the ambiguity returned with the US raid in Abbottabad, in which Bin Laden died in May this year.

The officials involved now dispute who is responsible for the confusion, and for the failure to kill him for so long.

One of the central arguments is whether the White House passed a separate and clear instruction to the CIA authorising them to plan to kill Bin Laden in the 1990s.

Richard Clarke, a counter-terrorism adviser to President Bill Clinton, says it did.

I want Bin Laden's head shipped back in a box filled with dry ice Cofer Black, Former head, CIA counterterrorist centre

"President Clinton did some extraordinary things," argues Clarke. "He broke a 25-year tradition and agreed that there should be a hit list. The United States government should sponsor the assassination of terrorists."

Then CIA Director George Tenet and others claim an instruction was passed down the ranks. But those lower down claim they never received it.

The suspicion in some quarters - denied by Tenet - is that he and other senior officials feared the consequences of the CIA being seen to get back into the assassination business.

In the 1970s, the CIA had faced widespread condemnation when it emerged that it had tried to assassinate foreign leaders - including Patrice Lumumba in the Congo and Fidel Castro in Cuba. Appearances before Congress scarred the agency deeply.

Submarines

In the late 1990s, the government authorised the US military to use lethal force if it was sure it could kill Bin Laden, and that there would be minimal civilian casualties.

Image caption The 9/11 attacks hardened US resolve to kill Osama Bin Laden

"The reality was that the White House - the president, the national security adviser, me - were asking the Pentagon to do it," says Richard Clarke.

"And the Pentagon - particularly the senior military leaders - were saying, 'No, too risky, don't have the capability, can't do it.'

"And then those same military leaders would turn round and tell their troops, 'The White House won't let us do it.'"

General Richard Myers, vice-chairman and then after 9/11, chairman, of the Joint Chiefs of Staff, says the Pentagon was never risk averse, but was not geared up in the years immediately after the Cold War for this kind of operation.

"We needed to be lighter, leaner, more flexible, more agile. We are that now. But we weren't in 2001 at all, and we certainly weren't in the four or five years before that when we knew where Bin Laden was."

The US eventually stationed two nuclear submarines off the Pakistani coast ready to launch cruise missiles if Bin Laden was located.

Michael Scheuer, head of the CIA Bin Laden unit, claims the intelligence was sometimes good enough to fire.

The problem was the CIA always knew where he was yesterday but not tomorrow Richard Clarke, White House counter-terrorism advisor

"The Sunday before Christmas in 1998, Bin Laden was staying overnight in the governor's palace in Kandahar. We knew he was there because we saw him go into the room, and we know which wing of the palace, which room he was in," recalls Scheuer.

"And the president and his advisers decided that 'Well, if we kill him, some of the shrapnel might hit a mosque that was about 300 metres away.'"

The operation was aborted.

Others dispute whether the intelligence was good enough. "The problem was the CIA always knew where he was yesterday but not tomorrow," argues Richard Clarke.

Regret

The 11 September 2001 attacks changed the calculus again. President Bush signed a secret authorisation to hunt down top al-Qaeda leaders.

Cofer Black, the tough-talking head of the CIA's counterterrorist centre, went in to brief the president.

"By the time we get through with these guys, we'll have flies walking across their eyeballs," he recalls telling President Bush. "And at that point you could see that he was a believer."

Black issued unusual orders to Gary Schroen, who was being sent out to Afghanistan to lead the CIA team.

Find out more The Hunt for Bin Laden is on BBC Radio 4 on Friday 15 July and Friday 22 July at 1100 BST. Listen again via the BBC iPlayer

"I want Bin Laden's head shipped back in a box filled with dry ice. I want to be able to show Bin Laden's head to the president," Schroen recalled Black telling him.

Black now maintains that this request was not borne out of a desire for a grisly trophy but to aid identification. "You have to have a compelling proof that you've been successful and it can't be, 'Well it looked like him.'"

The hesitation over being seen to engage in assassination extended to the final operation in Abbottabad.

"The operation was designed as a kill or capture operation," US Attorney General Eric Holder told me.

"It was not an assassination. If we could have captured him - if we could have taken him alive - that is something that we would have done."

But the account of what happened during the 38-minute raid changed markedly in the days after Bin Laden's death.

"He did resist the assault force. And he was killed in a firefight," a senior administration official said immediately after the operation.

However, it then emerged that Bin Laden was not armed and did not fire back. That has led to the widespread belief that the operation was "kill or capture" only in the most technical, legalistic sense.

"Had he indicated very clearly that he wanted to surrender, they were prepared to take him in that way," Holder argues.

But in the dead of night and in the chaos of the compound, a clear signal seems implausible. There can have been little doubt how it would end.

For some, like former FBI man Jack Cloonan, the satisfaction at seeing Bin Laden gone is tempered by a regret that there was never the chance to bring him to justice before a court.

"We had dreamt," he says, "of Bin Laden in an orange jump suit that said 'Metropolitan Correction Centre' on the back of it standing in the southern district of New York."

The Hunt for Bin Laden is on BBC Radio 4 on Friday 15 July and Friday 22 July at 1100 BST. Or listen again via the BBC iPlayer.