Tests on a failed parcel bomb sent on a US-bound cargo flight last month show it could have been designed to detonate over the eastern US, say UK police.
The bomb was found in a printer cartridge on a plane in a UK airport, after being posted from Yemen.
A second printer bomb, also sent from Yemen, was intercepted in Dubai.
The bombs were both addressed to synagogues in the US city of Chicago, and were claimed by al-Qaeda in the Arabian Peninsula (AQAP).
The UK bomb, intercepted at East Midlands airport, was discovered early on 29 October, following a tip-off from Saudi intelligence.
It was removed and "disrupted" by explosives officers about three hours before it was timed to detonate, British police said in a statement.
"If the device had activated it would have been at 1030hrs BST (0930 GMT) on Friday 29 October 2010," they said.
"If the device had not been removed from the aircraft the activation could have occurred over the eastern seaboard of the US."
The White House said the findings "underscore the serious nature of the attempted AQAP attack and the challenge we all face in trying to prevent or disrupt such attacks".
Spokesman Nicholas Shapiro thanked the UK authorities for the "highly professional nature" of their investigation, and said the US would continue to work closely with London - as well as with Saudi Arabia, United Arab Emirates and Yemen - to counter the threat posed by AQAP.
Last week, French Interior Minister Brice Hortefeux said one of the two bombs had been 17 minutes from detonating when it was discovered - he did not specify to which device he was referring.
Both the intercepted parcels contained the powerful plastic explosive PETN, which is difficult to detect.
They had been posted in the Yemeni capital Sanaa, and sent through the freight firms UPS and FedEx.
The UK bomb travelled via Dubai and is then thought to have passed through Cologne in Germany, before arriving at East Midlands airport, while the device found in Dubai is thought to have travelled by passenger jet via Doha.
UK officials says the crucial tip-off came from a former member of al-Qaeda Jabr al-Faifi, who had recently handed himself into the authorities.
AQAP - regional offshoot of Osama Bin Laden's militant network - claimed on Islamist websites last week that it had carried out the attempted attacks - the organisation vowed to continue attacking the US and its allies.
Security forces in Yemen are hunting for AQAP's suspected bomb maker, Ibrahim al-Asiri, in connection with the plot.
BBC Security Correspondent Frank Gardner says the news that the explosives were probably targeting the US is significant and underpins what had long been suspected.
The assumption is that AQAP would not have been able to predict the route the two bombs would take, but that their intention was to show they were capable of sending such explosives on a long distance journey, ending with an attack on the US.