Fears that terrorists with sophisticated and harder-to-detect body bombs will target U.S.-bound jetliners cast a cloud over July 4 Independence Day celebrations.
The heightened security alert and Hurricane Arthur spinning up the East Coast were both expected to disrupt travel.
The threat emerged as intelligence intercepts pointed to bomb-makers from Yemen-based al-Qaeda in the Arabian Peninsula linking up with the al-Nusra Front, an al-Qaeda affiliate in Syria. Al-Nusra counts among its jihadists, scores of Westerners with passports who don't need visas to enter the United States, according to U.S. security officials.
Thousands of passengers on transatlantic flights were subject to extra scrutiny – especially shoes and clothing – Thursday, apparently after intelligence indicating that al-Qaeda-linked bomb makers in Yemen may have perfected a wearable or internal bomb possibly using chemical detonators rather than conventional ignition sources.
New models of smartphones also came under intense security at checkpoints for passengers on U.S.-bound flights in Europe, the Middle East and Africa.
In Kampala, a special security clampdown – complete with spot searches on city streets by heavily armed soldiers – was underway in the Ugandan capital after the U.S. embassy there issued an unusually specific warning: "According to intelligence sources, there is a specific threat to attack Entebbe International Airport by an unknown terrorist group" on Thursday evening. Although no attack occurred Thursday, the airport remained on high alert. Entebbe is one of Africa's busiest airports for flights to European hubs and connections to the United States.
U.S. intelligence officials were reportedly concerned that Western jihadists, with European or North American passports and currently fighting in the Islamist-led Sunni insurgency in Syria, may serve as carriers for the al-Qaeda cell in Yemen headed by Ibrahim al-Asiri, the Saudi engineer who devised several non-metal bombs that have come perilously close to destroying airliners. Many Western European nationals can enter the United States without a visa, thus avoiding the extensive background checks that security officials regard as the best defence against a bomb-carrying terrorist managing to get on an airliner.
But even that safeguard has failed as bombs have become smaller, more sophisticated and harder to detect.
Umar Farouk Abdulmutallab, a Nigerian, wearing a powerful plastic bomb woven into his underwear, was overpowered by passengers as he attempted to detonate the device shortly before his Amsterdam-to-Detroit flight landed on Christmas Day in 2009. A few months later, a pair of Asiri-designed bombs hidden in printer cartridges were found on board U.S.-bound cargo planes.
Homeland Defence Secretary Jeh Johnson said foreign allies had been alerted. He ordered U.S. security agents abroad to impose "enhanced security measures in the coming days at certain overseas airports with direct flights to the United States."
Britain also tightened security in the wake of the U.S. alerts. Deputy Prime Minister Nick Clegg warned the delays and heightened screening would last indefinitely and were not "just a blip for a week."
Passengers boarding flights for the U.S. reported delays as carry-on baggage was subject to additional searches.
In a television interview, Mr. Johnson confirmed "a terrorist threat to this country that remains."
The warning and tighter security came just before the July 4 long weekend. Despite steamy heat, massive crowds were expected in Washington. But with the season's first named storm – Hurricane Arthur – expected to dump heavy rain and flood some coastal areas, Independence Day celebrations all along the Atlantic Coast were being altered.
In Boston, a concert by the Boston Pops was shifted to Thursday ahead of the storms. Along the Outer Banks in North Carolina, many celebrations were pushed back to Saturday or Sunday.
Ocean City in Maryland and Virginia Beach both announced their July 4 fireworks would be held on July 5.
In Washington, the National Parks Service said the massive display of 6,000 fireworks set up near the Lincoln Memorial would like up the Mall as scheduled Friday night unless there was heavy rain.
With a report from Associated Press