By Paul Reynolds
World affairs correspondent BBC News website
The admission by the US Navy that Iranian speedboats might not have been the source of an apparent threat to attack American ships in the Gulf is a significant move that raises new fears about the chances of unintended clashes in the region.
It has worrying similarities with the incident in 1988 when, in the same Strait of Hormuz, the USS Vincennes shot down an Iranian civilian airliner, having failed to monitor the radio traffic properly.
The crew of the Vincennes became wrongly convinced that the airliner, an Airbus with 290 people on board, all of whom died, was an Iranian fighter jet.
The Iranian government said that the destruction of the plane was done in full knowledge of what it was.
The US government later suggested that one factor at play on the Vincennes was a condition called "scenario fulfilment" in which military personnel are under such pressure that they expect and then execute a particular scenario, as if in an exercise.
Whether the same expectation was at play in this latest incident is not clear.
What is clear is that there are grave doubts about who uttered the warning picked up by the US ships. A deep voice was heard to say: "I am coming at you. You will explode after a few minutes."
The video released by the US implied that the warning was part of a series of transmissions to the ships from the Iranian craft.
It turns out that the warning was added onto the video. It was a radio recording made separately.
Experts say it could have come from another ship in the area or from a radio transmitter on shore. The channel used by the Iranian vessels to make their inquiries is an open one.
The Iranians later issued their own video, in which one of their sailors, in a much higher and quite different voice from the one which issued the "warning", asks the US ships who they are and what course they are on.
He gets a dusty reply that the US vessels are in international waters.
The Iranian video does not show their boats buzzing close (200m or so) to the Americans.
The US said that in any case the Iranian speedboats acted aggressively. Iran's version is that this was a routine check by its sailors.
Beyond the propaganda
This goes beyond the back and forth of a propaganda battle, in which once again the Iranians show themselves to be masters.
It recalls the ease with which they ran rings round the Royal Navy when they captured British sailors and marines off Iraq last year, exposed them to damaging publicity before releasing them with handshakes by their president.
The real concern is that a possibly misread radio transmission should be at the heart of this incident, nearly 20 years after the Vincennes disaster.
Tensions between Iran and the US have diminished recently following the US National Intelligence Estimate that Iran is not building a nuclear weapon.
But there are still serious points of potential conflict between the two, with Iran always determined to exert its influence in what it insists on calling the Persian Gulf and the US maintaining strong naval forces there in international waters and in the waters of its Gulf Arab allies.
Iran is also subject to UN sanctions, having refused the demands of the Security Council to suspend its enrichment of uranium.