Officials isolated UPS cargo planes that landed at Philadelphia and Newark airports on Friday morning because they were carrying packages that appeared to have the same origin as the suspicious device discovered in Britain. A UPS truck in Brooklyn was also stopped and checked on Friday. None of the other packages that have been examined have been found to pose any danger.
The White House was told about the potential threat on Thursday night after authorities identified and examined two suspicious packages, one in London and one in Dubai, according to Robert Gibbs, the White House press secretary.
"Both of these packages originated from Yemen," Mr. Gibbs said in a statement. "As a result of security precautions triggered by this threat, the additional measures were taken regarding the flights at Newark Liberty and Philadelphia International Airports."
At East Midlands Airport in Donington, England, near Birmingham and about 100 miles north of London, a UPS cargo sorting facility was evacuated after a suspicious package that originated in Yemen was discovered in the predawn hours of Friday.
In the package, officials found a toner cartridge that appeared to have been tampered with in a way that made it resemble an improvised bomb. But tests for the presence of explosives were negative, a law enforcement official said, speaking on the condition of anonymity because the investigation was continuing.
The official did not have specific details on what was found, but added that it was "not an explosive." The Associated Press and CNN reported that the toner cartridge had wires and an electronic circuit board attached to it and that it was covered with a white powder.
An alert at the East Midlands airport was raised at 3.30 a.m. local time and later lifted, only to be reimposed at 2 p.m. on Friday. Four hours later, a wide area around the cargo center remained cordoned off, with police and counter-terrorist units searching the area.
A statement posted on the UPS web site said the company was cooperating closely with authorities in Britain and the United States. "Because these incidents are still being investigated, we don't have any further details." the statement said.
There were reports that one or more suspicious packages from Yemen was addressed to a synagogue in Chicago. On Friday, federal officials warned synagogues in the Chicago metropolitan area to be on alert, said Linda Haase, associate vice president of the Jewish Federation of Metropolitan Chicago.
"We were notified about this earlier this morning," Ms. Haase said in an interview. "We are taking appropriate precautions and we're advising local synagogues to do the same."
Shortly after noon, a spokesman for Newark Liberty International Airport, Steve Coleman, said the package that officials sought on the plane there "has been cleared."
Mr. Coleman said that the item in question was examined in Building 350, the UPS building at the airport, and "was deemed non-threatening."
At a press briefing, New York Police Commissioner Raymond W. Kelly said investigators had examined a package on a UPS delivery truck at the Chase Bank branch at the Metrotech complex in Brooklyn. The package had come from Yemen and passed through Kennedy Airport, Commissioner Kelly said, and it fit a pattern of such packages. It turned out to be an envelope containing receipts, he said, and posed no threat; neither did another package on the truck that looked similar, he said. Mr. Kelly gave little detail about how the packages came to the attention of the department, except to say their origin in Yemen was a factor.
CNN reported that Philadelphia Fire Department officials had examined a package found on a UPS cargo plane isolated at the airport there, and found it to be harmless.
American military officials have increased their support to Yemen after a Nigerian man suspected of training with Al Qaeda in Yemen tried and failed to explode a bomb hidden in his underwear on a Detroit-bound jetliner on Christmas Day.
The alert in Britain came just two days after the country's transport minister said he would listen sympathetically to demands from airlines that security measures for passengers at British airports be eased.
That followed a speech by Martin Broughton, the chairman of British Airways, who criticized the United States for "redundant" checks like screening passengers' shoes and requiring laptops to be removed from carry-on bags at checkpoints.
Mr. Broughton said there was no need to "kowtow to the Americans every time they want something done.
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