Authorities on three continents thwarted multiple attacks that triggered worldwide fears of a major new al Qaida terror campaign.
Yesterday's attacks were aimed at the US from Yemen, in two explosive packages packed aboard cargo planes and addressed to Chicago synagogues.
US president Barack Obama called the co-ordinated attacks a "credible terrorist threat" and US officials said they were increasingly confident that al Qaida's Yemen branch, the group responsible for the failed Christmas Day Detroit airliner bombing was responsible.
Mr Obama's anti-terror chief John Brennan warned last night: "The US is not assuming that the attacks were disrupted and is remaining vigilant."
One of the packages was found aboard a cargo plane in Dubai and the other at East Midlands Airport.
Preliminary tests indicated the packages contained the powerful industrial explosive PETN, the same chemical used in the Detroit attack, US officials said. The tests had not been confirmed.
In the US, cargo planes were searched up and down the Eastern Seaboard, and an Emirates Airlines passenger plane was escorted down the coast to New York by American fighter jets.
No explosives were found aboard those planes, although the investigation was continuing on at least two.
Mr Obama's sobering assessment, delivered from a White House podium, unfolded four days before mid-term US elections in which discussion of terrorism has played almost no role. The president went ahead with weekend campaign appearances.
The terrorist efforts "underscore the necessity of remaining vigilant against terrorism", the president said.
While he said both packages that contained explosives originated in Yemen, he did not explicitly assign blame to al Qaida, which is increasingly active in that Arab nation and long has made clear its goal of launching new attacks on the United States.
Authorities in Dubai intercepted one explosive device. The second package was aboard a plane searched at East Midlands Airport and officials said it contained a printer toner cartridge with wires and powder.
Mr Brennan said the devices were in packages about the size of a bread box.
Mr Brennan said al Qaida's Yemen operation was the most active of its franchises and said anyone associated with the group was a subject of concern.
Radical US-born Muslim cleric Anwar al-Awlaki, who now is in hiding in Yemen, is believed to have helped inspire recent attacks including a shooting at Fort Hood, Texas, in which 12 soldiers were killed, the Times Square bombing attempt and the Detroit airline bombing attempt.
Another American hiding in Yemen, Samir Khan, has declared himself a traitor and has helped produce al Qaida propaganda.
Mr Brennan later said the explosives "were in a form that was designed to try to carry out some type of attack", but provided no further details.
"The forensic analysis is under way," he said. "Clearly, from the initial observation, the initial analysis that was done, the materials that were found in the device that was uncovered was intended to do harm."
Intelligence staff had been monitoring a suspected plot for days, officials said. The packages in the UK and Dubai were discovered after Saudi Arabian intelligence picked up information related to Yemen and passed it on to the US, one official said.
Mr Brennan later issued a statement thanking the Saudis for their help and praising British and United Arab Emirates officials for helping prevent violence.
US intelligence officials warned last month that terrorists hoped to send chemical and biological materials as part of an attack on America and other Western countries using the postal system.
The alert came in a September 23 bulletin from the Department of Homeland Security that was obtained by The Associated Press.
In the hours following the discoveries, Yemeni officials and Scotland Yard were investigating and the US issued a 72-hour ban on all cargo from Yemen.
US authorities searched aircraft in Philadelphia, Pennsylvania; Newark, New Jersey; and New York City.
Since the failed Detroit bombing attempt, Yemen has been a focus for US anti-terror officials. Before that attack, the United States regarded al Qaida's branch in Yemen as primarily a threat in the region, but not to the US.
The Yemen branch known as al Qaida in the Arabian Peninsula has since become a leading source of terrorist propaganda and recruiting. Authorities believe about 300 al Qaida members or cells operate there.
The Yemeni government has stepped up anti-terror operations with help from the US military and intelligence officials. Mohammed Shayba, general director of the state airline's cargo department, said the government was conducting an investigation.
"Those in charge are in constant meetings and they are investigating and taking the issue seriously," he said.