By Paul Reynolds
World affairs correspondent, BBC news website
In October 2005, the then British ambassador to Iraq William Patey told reporters in London that Iran had been supplying technology used to kill British troops in Basra.
US photo of bomb damage from an EFP - explosively formed penetrator
He said he had complained to the Iranian ambassador in Baghdad about it.
The claim was that elements connected to the Shia militia in the south, the Mehdi army, had been using specially shaped charges, in which the force of the explosion is directed narrowly in one direction, thereby enabling it to penetrate armoured vehicles.
No evidence was produced, other than a suggestion that the Iranian-supported Lebanese group Hezbollah had also used such charges, so the common origin had to be Iran.
US officials have made similar claims over the last year. General George Casey, the then US commander in Iraq, said so in June 2006.
In a briefing in Baghdad on Sunday, US military and intelligence officers finally laid out their evidence.
The question has to be asked as to why it has taken at least 14 months for this to happen.
So, why now?
If you take the claims at face value, the reason is that only now has the evidence become substantial enough to be made public. The number of attacks is said to have grown as well, so that is another explanation put forward for going public now. A trend has been identified about which information should be given.
According to this position, there is nothing sinister about the timing of the claim. It is the result of an evidence-based process which has only now reached the stage of producing a result. And after all, reporters have been asking for this evidence for months.
There are other possibilities as well.
For a start, the fear among some is that the US is softening up world opinion for an attack on Iran. Such an attack would be aimed at Iran's nuclear facilities.
At the moment, the US lacks a casus belli and by claiming that Iran is responsible for killing USA troops, it could be laying the groundwork for a 'self-defence' justification, according to this theory.
The new chairman of the Senate Intelligence Committee, Senator John Rockefeller said recently: "To be quite honest, I'm a little concerned that it's Iraq again."
There is also the fact that the US is launching its 'surge' policy of moving extra troops into Baghdad. These claims are being made against Shia militias, including the Mehdi army, one of the main targets of the latest policy.
Blaming Shia Iran for supporting Iraqi Shia militias makes it easier for the US to sell that policy at home and abroad.
Then there is the old tactic of blaming someone else for your own problems.
Many people will not distinguish between the Shia militias that Iran is said to supply - and which have ties to the Iraqi government - and the Sunni insurgents who have been the cause of much of the violence.
The allegedly Iranian supplied bombs are said to have caused the deaths of 170 American soldiers, but overall 2497 soldiers have been killed in hostile incidents, most of them at hands of the Sunnis.
The claim serves the purpose of helping to lay the blame for the whole insurgency at Iran's door.
There are also other possible reasons for this timing.
The UN Security Council has laid down that Iran must suspend its enrichment of uranium by 21 February. If it does not, and if the International Atomic Energy Agency confirms this, the resolution says that further economic sanctions will be considered.
The US is preparing to argue for tougher sanctions, so making claims against Iran over Iraq might help it in its arguments that Iran is a threat.
On the wider front, the Bush administration is engaged in a campaign against the Iranian government in order to isolate it and eventually maybe see its end under internal pressure from the Iranian people.
The latest claims against Iran could be a part of that campaign.
What of the claims themselves?
They are based on physical evidence, from bombs and their effects. The bombs now even have their own name and abbreviation - explosively formed penetrators or EFPs.
Previously they had been lumped in the generalised description of IEDs - improvised explosive devices.
The implication is that now they are less improvised and more planned.
They are said to be provided by Iran in kit form and to be smuggled across the often-open border.
However the officials who presented the evidence could not make a direct link to Iran.
"The officials said such an assertion was an inference based on general intelligence assessments," stated the New York Times.
They did make much of the detention in Irbil of five Iranians who were said to be members of the Quds force of the Iranian revolutionary Guards.
The Quds (referring to Jerusalem) force was said by the US officials to be controlled directly by the "highest levels of the Iranian government".
That last statement is significant in that the US is now making a charge against the Iranian government itself, not just against its agents.
Against the inference that this all comes from Iran is the concept that Iraqis themselves would be capable of copying a design and therefore do not need to get bombs from Iran.
And there have been a number of news reports over the last year expressing scepticism, even among military personnel, about the link to Iran.
The Washington Post reported last October that British troops in the south doubted the claim.
A year ago, the London Times said that British officers in Basra had stopped making any such claim, saying only that the technology matched bomb-making found elsewhere in the Middle East, including Lebanon and Syria.