The first public signs emerged last week in a French television interview. France's interior minister, Brice Hortefeux, told a weekly talkshow that Saudi Arabian intelligence was warning that al-Qaida in the Arabian Peninsula (Aqap), which has its headquarters in Yemen, was planning a new wave of attacks and urged Europe to be on its guard.
Hortefeux said the warning had been received "in the last few days", prompting speculation that Aqap was flexing its muscles after the recent arrests of several high-profile terrorists.
It was not the first time Saudi intelligence had warned of a plot threatening Europe. The Saudi leadership insists it passed information to the UK relating to the London bombings of 2005, although MI5 maintains that what was predicted was "materially different from the attacks that took place on 7 July".
But on Friday, the credibility of the latest warning appeared to be confirmed. It emerged that explosive devices were found in two packages bound for the US from Yemen, one in the hold of a UPS cargo plane that had landed at East Midlands airport, the other on a FedEx plane in Dubai.
The discovery caused huge disruption and confusion, with the US scrambling fighters to accompany a passenger jet and mobilising explosive experts to search cargo planes. There was speculation that the parcels, which were addressed to two synagogues in Chicago, may have been a test of cargo-screening procedures. "This may be a trial run," one US official said. British intelligence sources confirmed they were also exploring this angle – and checking whether other flights may also have been used to carry suspect devices out of Yemen.
But as compelling evidence emerges that the devices were intended to go off on board the cargo planes, rather than at the synagogues to which they were addressed, experts warn they may represent the start of a trend for low-investment, high-impact attacks that are difficult for intelligence agencies to detect.
The explosives were built into printer cartridges, prompting concerns that terrorist groups are coming up with innovative new ways to launch attacks on foreign soil. Jane Harman, a Democrat on the House homeland security committee, who was briefed by the US Transportation Security Administration, said one device used a mobile phone as a detonator and the other had a timer.
Ben Venzke, chief executive of the intelligence agency IntelCenter, said that earlier in the year Aqap's official Arabic-language magazine, Sada al-Malahim, had issued instructions on how to build IEDs (improvised explosive devices). "The article provides insights into how the group approaches IED design and creates devices for specific targets and operations," he said. "The creation of devices built into toner cartridges fits within this philosophy and would not be surprising to see coming out of Aqap."
A White House counter-terrorism adviser, John Brennan, acknowledged that al-Qaida had attempted to adapt to US security measures in the wake of the 11 September attacks in 2001, which have largely focused on commercial – not cargo – aircraft. "Clearly they are looking for vulnerabilities in our system," Brennan said, adding that Aqap was "the most active operational franchise" of al-Qaida outside its traditional Pakistan and Afghan bases.
According to US officials, initial tests have indicated the devices contained PETN, the same powerful explosive used by Umar Farouk Abdulmutallab, a Nigerian who studied in the UK and was radicalised in Yemen, in his failed attempt to blow up a plane over Detroit last Christmas.
Dubai police have also confirmed that there are "features similar to previous attacks carried out by terrorist organisations like al-Qaida", adding that their tests showed the printer cartridge also contained lead azide, an explosive compound used in bomb detonators.
There are only a handful of international shipping facilities in Yemen, all of which are heavily scrutinised, prompting questions about how the packages were loaded on to the planes without triggering a security alert. Terror expert Dr Sally Leivesley said it appeared the powdered toner may have been used as a means of evading screening. "It's a step-jump change in terms of threat to aviation and it's extremely serious," she said. "These devices can be put on board anywhere."
Security at Yemen's Sana'a airport has been tightened significantly after US officials raised concerns that travellers and luggage were not being carefully screened. Only a few years ago, Yemeni passengers were allowed to travel on internal flights wearing their jambiyas, a traditional curved dagger. Today, local passengers say that airport security staff apologise to them for being frisked and for searching their bags. It is considered a violation of dignity in Yemen's conservative Muslim society, particularly when female passengers are searched.
Yemeni officials insisted security at their airport is up to international standards and disputed claims the explosives originated in Yemen. "There is no proof that the package came through the airport and I confirm that the package was not from Yemen," Fiaz Gazali, a senior security official at Sana'a airport told the Observer. "UPS or FedEx has not landed a single plane in our airports for a long time."
However, Scotland Yard sources confirmed the package removed from the plane at East Midlands airport originated from Yemen. The package was in a container transported from Yemen to Cologne, Germany, which was then transferred to the flight that stopped at East Midlands on the way to Chicago.
In a public address, President Barack Obama said: "Although we are still pursuing all the facts, we do know that the packages originated in Yemen." Obama added: "We also know that al-Qaida in the Arabian Peninsula, a terrorist group based in Yemen, continues to plan attacks against our homeland, our citizens, and our friends and allies." Mindful of the political implications of a plot so near to voting day on Tuesday, Obama used the address to reassure America that "we will not waver in our resolve to defeat al-Qaida and its affiliates and to root out violent extremism in all its forms".
This year Aqap, which boasts some 300 members according to intelligence officials, has co-ordinated an unsuccessful suicide bomb attack on the British ambassador to Yemen, Timothy Torlot, and a rocket-propelled grenade attack on his deputy, Fionna Gibb.
Only last week, the head of MI6, John Sawer, warned that the radical cleric Anwar al-Awlaki continued to operate out of Yemen, broadcasting al-Qaida propaganda over the internet. Awlaki, who was born in New Mexico, is believed to have mentored a US army psychiatrist charged with killing 13 soldiers in a November 2009 shooting rampage at Fort Hood, Texas. He is also credited as the inspiration behind Faisal Shahzad, who pleaded guilty to the 2010 Times Square car bombing attempt.
Speaking to the Observer, Yemen's deputy prime minister for defence affairs, Rashad al-Alimi, said security forces knew of Awlaki's whereabouts and have him surrounded, but stressed the cleric would not be turned over to US authorities. "If we capture Awlaki, we will not hand him to the US, and we will ask for evidence against him and give him a fair trial in Yemen," Alimi said.
Salem Abdu, a member of Awlaki's tribe, said it was working with the government to hunt down suspected al-Qaida operatives, a tactic similar to the US military's policy of persuading Sunni tribes in Iraq to turn against al-Qaida. "We want peace in this country and we are after anyone who wants chaos in our region," he said. "We are working as security personnel for the government hunting for terrorists. We do not get much assistance, but we are helping our country improve."
A report by the foreign affairs thinktank Chatham House last week warned that Yemen is seen as a safe haven for terrorists and is increasingly playing host to trained militants, including veterans of the Afghanistan and Iraq conflicts. "There are questions over the capability of the authorities to engage in counter-terrorism operations as allies – or proxies – of western governments," the report claimed.
"Fears are sharpened by the fact that al-Qaida is thought to be recruiting and training US and European citizens at camps in Yemen and Somalia – including converts to Islam and so-called "non-traditional" recruits – to launch attacks against American targets within the Middle East and beyond."
Political analysts in Sana'a fear the heightened security concerns triggered by the latest plot will deal a serious blow to the country's already desperate economy. "They wanted media and they're getting it. It doesn't really matter if the bomb exploded or not," said Abdel Ghani Aryani. "The damage to Yemen's economy will be massive. We assumed al-Qaida could not destabilise the regime, but the cost of doing business here is getting higher and higher."
As Obama pledged security would be increased for air travel for as long as necessary, concerns about other devices prompted major security alerts across US cities. All direct flights from Yemen to the UK were suspended. Theresa May, the home secretary, emerged after chairing a meeting of Cobra, the UK government's emergency planning committee, to say: "I have agreed with the transport secretary to take immediate action to stop the movement of all unaccompanied air freight originating in Yemen and moving into or through the UK. We are in contact with the transport sector about this. Direct cargo and passenger flights from Yemen were suspended for security reasons in January this year, following the earlier attempt to bomb an aircraft destined for Detroit."
The plot is likely to silence calls for security measures to be relaxed. Last week, the British Airways chairman, Martin Broughton, suggested some parts of the security programme were now "completely redundant", adding that there was no need to "kowtow to the Americans every time they wanted something done."
But one of May's predecessors, Lord Reid, said the latest plot was a wake-up call. "We've got to get away from this idea that if we go a period of time without an incident like this that we can reduce vigilance and security at airports."