U.S. counterterrorism officials say 2009 has turned into the year of homegrown jihad, with the unmasking of the most serious suspected terror plots involving Americans in about five years.

U.S. investigators are still trying to determine what drew five young Americans to travel last month to Pakistan, where local authorities allege they had sought to join extremist groups that have attacked U.S. soldiers in Afghanistan. U.S. investigators have interviewed some of the men, but haven't verified the information Pakistani officials have released on the case.

The surge in alleged terror cases has raised concerns among counterterrorism officials. Some officials say young men have been swayed by the escalating war in Afghanistan and Pakistan, as well as intensifying Internet recruiting of Westerners by extremist groups.

At a Senate Homeland Security and Government Affairs Committee hearing last month, experts on terrorism cited the recent cases as evidence that the threat of radicalization, long an issue in Europe, has become a major concern in the U.S.

In September, terrorism investigators trailed Najibullah Zazi, a 24-year-old Afghan-born U.S. resident, as he made his way from Denver to New York City in what they later alleged was an aborted plot to carry out attacks using explosives made from beauty supplies.

A month later, investigators arrested David Headley, a U.S.-born son of a Pakistani father and American mother, and charged him with plotting with Al Qaeda-linked Pakistani militants to attack a newspaper in Denmark that printed satirical cartoons of the Prophet Muhammad. In recent weeks, investigators alleged that Mr. Headley told them he helped a Pakistani group, Lashkar-i-Taiba, scout locations for the deadly 2008 Mumbai terror attack.

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