WASHINGTON — Calling it a "credible terrorist threat," President Barack Obama said apparent explosive material was found on two U.S.-bound packages from Yemen, triggering searches of flights with other packages from Yemen and an investigation into whether al-Qaida was behind a new terror plot.
Sources told NBC News that both packages were actually laser printers that contained toner cartridges that had been tampered with, showing extruded wires and white powder. The packages, addressed to Jewish institutions in Chicago, were found in Britain and Dubai on Thursday night.
"An initial examination of those packages has determined that they do apparently contain explosive material," Obama told reporters.
"The United States is not assuming that the attacks were disrupted and is remaining vigilant," added White House counterterrorism adviser John Brennan.
The devices were "in a form that was designed to try to carry out some kind of attack," he said. "Clearly, from the initial observation, the initial analysis that was done, the material that was found ... was intended to do harm."
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Brennan said the two packages had "been made inert."
Brennan also thanked Saudi Arabia for "developing information" about the packages. The tip that led to the discovery of the packages came from the Saudi Arabian authorities, sources said.Video: Watch comments by counterterror adviser John Brennan (on this page)
PETN apparently detected
U.S. officials said initial indications are the packages contained PETN, a chemical that was also a component of shoe bomber Richard Reid's explosive in 2001 and last year's foiled Christmas Day airliner bombing attempt.
PETN is the primary ingredient in detonating cords used for industrial explosions and is popular among terrorist groups.
U.S. officials told NBC News the amount of explosives in one of the packages was about five times greater than quantity carried by the would-be Christmas Day bomber. That would make it slightly less than a pound, but that's a significant amount.
It was unclear how the devices were to be set off.
An electronic component seen in an image of one intercepted shipment appeared to be a printed circuit board from a dissassembled cell phone, said Olivier Clerc, a hardware application engineering manager contacted by CNN.
Clerc did not say how the circuit board would be used in the captured device, but terrorist groups have triggered bombs with cell phones, CNN said.
Investigators were also looking at recent shipments from Yemen to the U.S., particularly to Chicago, to see if whoever sent the packages had tried earlier to do a test run, accprding to NBC News.
Homeland Security said in a statement it was increasing cargo screening and boosting security at airports. Shipments from Yemen were also suspended until at least Nov. 1.
In the U.S., cargo planes were searched up and down the Eastern Seaboard, and an Emirates Airlines passenger jet was escorted down the coast to New York by American fighter jets.
No explosives were found aboard those planes, though the investigation was continuing on at least two.
The Yemeni government expressed astonishment at reports linking it to the explosive packages. In a statement, the government warned against "rush decisions in a case as sensitive as this one and before investigations reveal the truth."
Yemen is the home of al-Qaida in the Arabian Peninsula, the offshoot branch that claimed responsibility for an attempted bombing of a U.S.-bound airliner last Christmas.
While Obama didn't specifically accuse Yemen's al-Qaida branch, Brennan called it the most active al-Qaida franchise and said anyone associated with the group was a subject of concern.
The radical U.S.-born Muslim cleric Anwar al-Awlaki, who now is in hiding in Yemen, is believed to have helped inspire recent attacks including the Fort Hood shooting, the Times Square bombing attempt and the failed Detroit-bound airliner bombing last Christmas Day. Another American hiding in Yemen, Samir Khan, has declared himself a traitor and has helped produce al-Qaida propaganda.
One of the packages was found at a FedEx facility in Dubai, the other aboard a cargo plane in East Midlands, north of London. Officials said they contained a printer toner cartridge with wires and powder. Brennan said the devices were in packages about the size of a breadbox.
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In Chicago, synagogues were warned to be on alert Friday.
Secure Community Network, part of the Jewish Federation of North America, said Homeland Security advised it to be on a look out for suspicious packages "that can be larger than a shoebox" from Yemen and other Middle East locations. The SCN sent out alerts to all Jewish communities and about 70 percent of synagogues across the United States. Security was heightened at all Jewish institutions in the Chicago area.
Authorities also searched three UPS planes and their cargo at Newark and Philadelphia airports, and an Emirates Airlines passenger jet was escorted down the coast to New York by American fighter jets. No explosives were found aboard those planes.
Al-Qaida active in Yemen
The United States has stepped up its training, intelligence and military aid to Yemen after the failed Christmas Day plot, for which the Yemeni wing of al-Qaida claimed responsibility.
Al-Awlaki, who now lives in hiding in Yemen and is an al-Qaida leader there, is believed to have helped inspire recent attacks including the Fort Hood shooting, the Times Square bombing attempt and the failed Detroit airliner bombing last Christmas Day.
The accused Christmas Day bomber, Umar Farouk Abdulmutallab, has told U.S. investigators he received the explosive device and training from al-Qaida militants in Yemen.
Yemen has been trying to quell a resurgent branch of al-Qaida, which has stepped up attacks on Western and government targets in the Arabian Peninsula country.
One official said intelligence personnel had been monitoring a suspected plot for days. The packages in England and Dubai were discovered late Thursday after a foreign intelligence service picked up information related to Yemen and passed it on to the U.S., this official said.
The Associated Press and Reuters, as well as NBC's Pete Williams and Robert Windrem, contributed to this report.