Thousands of foreign fighters have bolstered the ranks of militant groups in Syria and Iraq in recent months, according to U.S. officials, driving fresh concern about potential terrorist plots aimed at the United States or its allies.
The surge, as well as intelligence that Yemeni terrorists have developed a powerful cellphone bomb designed to avoid detection at airports, is behind the Obama administration's increasingly urgent warnings that a European or American passport holder might try to take down a passenger jet or plan other deadly mayhem.
As many as 10,000 foreign fighters — a third more than in February — have joined the militias seeking to overthrow Syrian President Bashar Assad and the Islamic State fighters who have swept across northern Iraq, said a counter-terrorism official who spoke on condition of anonymity to discuss classified assessments.
As many as 3,000 of them hold European or other Western passports and thus can travel easily across most borders. Several dozen, perhaps as many as 100, hold U.S. passports, and officials say that number is growing.
The potential partnership between some of those Westerners and sophisticated bomb makers from the Yemen-based Al Qaeda in the Arabian Peninsula, long viewed as the terrorist network's most dangerous offshoot, has raised alarm in Washington and other capitals.
"It's something that gives us really extreme, extreme concern," Atty. Gen. Eric H. Holder Jr. told ABC News last week. "In some ways, it's more frightening than anything I think I've seen as attorney general."
FBI Director James B. Comey told reporters in Washington that the threat "keeps me up at night." He sees the region as a "launching ground" for potential Sept. 11-style mass-casualty attacks in this country.
Two U.S. citizens have been arrested on terrorism-related charges since April before boarding flights to Turkey, suspected of being on their way to help the militants. One is said to have paid his way with his federal tax refund. In May, a Florida man became a suicide bomber in northern Syria.
"It's the largest number of Western fighters we have ever seen in a jihadist theater," said Seth G. Jones, a former U.S. counter-terrorism official now with the Rand Corp. think tank.
"The scale of this is huge," said Daniel L. Byman, professor of security studies at Georgetown University in Washington.
Some of the Western militants are joining Al Nusra Front, an Al Qaeda affiliate that is focused on defeating the Assad government in Syria. Others have rallied to the Islamic State, which broke away from Al Qaeda and has seized territory in eastern Syria and northern Iraq.
Although the rebels in Syria are struggling, the stunning battlefield success of the Islamic State fighters, and its calls for an Islamic caliphate, have energized recruiting efforts for all the militant groups. Several have used YouTube videos of battles and updates posted to Twitter in English to draw supporters to their ranks.
So far the Islamic State has focused only on capturing and consolidating territory in Syria and Iraq, but that could change. The Sunni Muslim group, recently known as the Islamic State of Iraq and Syria, or ISIS, is so innovative and is growing so quickly that one U.S. national security official called it the "Silicon Valley" of militant organizations.
"The concern is not what is happening today, but in four or five years," said the official, who spoke on condition of anonymity to discuss classified assessments. "If they get their own state, take on new projects, turn from looking inward to outward ... we want to get in front of it."
The Transportation Security Administration said early this month that it had asked some overseas airports to stiffen security measures, especially checking cellphones and other electronic devices to make sure they're not concealing explosives.
Intelligence agencies also have stepped up screening of passenger manifests of flights in and out of Turkey, the main entry point to Syria, officials said. In many cases, would-be fighters take a bus to the border and walk across.
A special counter-terrorism task force of FBI agents and federal prosecutors is investigating dozens of Americans who have traveled or who officials believe intend to travel to Syria to aid the militants, officials said.
"We're spending a tremendous amount of time and effort trying to identify those who go [to Syria] so we can know who they are when they come back," Comey told reporters.
The task force leader, veteran prosecutor Stephen Ponticello, joined Holder in Europe this month for meetings with allies that focused on the threat. Holder urged them to share intelligence on suspects, to conduct more undercover investigations and to enact stronger laws criminalizing support for terrorist groups.
The expanding conflict has become "a cradle of violent extremism," Holder said in a speech in Norway. "But the world cannot simply sit back and let it become a training ground from which our nationals can return and launch attacks."
With French and British citizens forming the largest bloc of Western passport holders in Syria, authorities in London and Paris already have begun to crack down.
Between January and March, at least 40 people in Britain were arrested on charges related to supporting militant groups in Syria, officials said. The British Home Office, empowered by a law Parliament passed in May, has stripped at least 20 people of their citizenship for suspected terrorism support or disloyalty, British officials said.
France is considering giving authorities additional power to block militant websites that seek to recruit foreign fighters, and to strengthen the ability of police to stop French citizens from traveling abroad to fight.
Americans have joined the conflict in Syria since it began in 2011, and at least one was killed. But their role as foreign fighters was little noticed until this May, when a 22-year-old Florida man blew himself up in a truck packed with explosives in northern Syria, killing more than a dozen government soldiers.
Born in West Palm Beach, Moner Mohammad Abusalha had an American mother and a Palestinian father. Known as "Mo," he liked to play basketball, was a fan of the Miami Heat and thought of a career as a physical therapist's assistant.
Abusalha went to Jordan in 2012, telling his family he would study nursing. He instead crossed into Syria and spent two months in an Al Nusra Front training camp, U.S. officials said.
Some Arabic speakers refer to one another by the name of their oldest child, but Abusalha had no children. He loved cats, though, so fighters called him "Abu Hurayra," which means father of kittens.
After his death, Al Nusra Front posted a propaganda video online that shows Abusalha stroking a gray and white cat and smiling. The video then cuts to a truck, with Abusalha apparently driving, approaching a building. Both are demolished moments later in plumes of fire and smoke.
The Americans arrested in recent months followed a similarly unlikely path.
Michael Wolfe, a native of Houston, told undercover FBI employees he would use a $5,000 federal income tax refund to take his wife and two children to Turkey and that he would then join the fighting in Syria, according to court documents. He would give any leftover cash to his mother-in-law, he said.
Wolfe, who has slicked-back black hair and a long goatee, was arrested in June by FBI agents at George Bush Intercontinental Airport in Houston as he prepared to board the first in a series of flights to Turkey.
Wolfe pleaded guilty in June to one count of attempting to provide material support and resources to a foreign terrorist organization. He faces a maximum sentence of 15 years in prison and a $250,000 fine.
Two months earlier, Shannon Maureen Conley, a 19-year-old nurse's aide from Arvada, Colo., was arrested at Denver International Airport as she prepared to fly to Adana, Turkey, less than 100 miles from the Syrian border.
Once she got to Syria, according to the charging documents, Conley planned to marry a 32-year-old Tunisian militant whom she had met on the Internet. She planned to use her medical training to help his cause.
Her plans were disrupted after a Faith Bible Chapel pastor in Arvada told local police in November that a young woman was acting suspiciously. Colorado state police called the FBI after Conley told an officer she intended to go overseas to wage jihad, according to the criminal complaint.
Conley told the FBI that she would engage in combat "if absolutely necessary" and that she had briefly joined the U.S. Army Explorers, a nonprofit youth program, to learn basic military skills. FBI agents and Conley's parents could not persuade her to change her mind, the documents state.
She was arrested at the airport on April 8. FBI agents who searched her luggage found DVDs labeled "Anwar al-Awlaki," the name of the American-born cleric and leader of Al Qaeda in the Arabian Peninsula who was killed by a U.S. drone strike in 2011. Recordings of Awlaki's English-language speeches are seen as powerful recruiting tools for Al Qaeda.
She pleaded not guilty on April 22 to a single count of conspiracy to provide material support to a designated foreign terrorist organization. If convicted, she faces a maximum penalty of five years in prison and a $250,000 fine.