US Official Says Security Threat Harder to Tackle

* Militants aim at broader range of potential targets

* Small-scale attacks are harder to detect

* Efforts to recruit Americans intensifying

By David Morgan

WASHINGTON (Reuters) - U.S. authorities are having a harder time detecting terrorism threats on American soil, top U.S. officials said Wednesday, more than nine years after the Sept. 11 attacks thrust the United States into a global struggle with Islamist militancy.

The officials, including Homeland Security Secretary Janet Napolitano and FBI Director Robert Mueller, said at a Senate hearing that al Qaeda and its affiliates are increasingly plotting small-scale attacks on the United States that can be carried out with greater frequency against a broader range of targets.

Militants have also intensified efforts to recruit American operatives in hopes of penetrating the U.S. security cordon.

"The threat is evolving in several ways that make it more difficult for law enforcement or the intelligence community to detect and disrupt plots," Napolitano said in written testimony submitted to the Senate Committee on Homeland Security and Government Affairs.

"The threats come from a broader array of groups and regions. It comes from a wider variety of harder-to-detect tactics. And it is aimed at harder-to-secure places than before," she said.

The document was filed before a hearing at which Napolitano was scheduled to appear alongside Mueller and National Counterterrorism Center Director Michael Leiter.

Mueller told lawmakers that a main concern now is from al Qaeda affiliates in Pakistan, Afghanistan, Yemen, Somalia and Iraq. The militant network's central leadership in Pakistan, headed by Osama bin Laden, is now at its weakest point since 2001 due to ongoing U.S.-Pakistani counterterrorism operations, according to officials.

The Sept. 11 attacks on New York and Washington killed about 3,000 people in 2001, prompting U.S. officials to fear that al Qaeda was plotting other mass casualty events, possibly including the detonation of a nuclear device.

But Napolitano said the failed May 1 car bombing in New York's Times Square and the Dec. 25, 2009, attempt to bomb a Detroit-bound airliner showed a shift toward attacks that require less extensive planning and coordination.

Both attacks caught U.S. authorities off guard.

As a result, Napolitano warned that the United States faces a growing threat from roadside bombs like those used against American troops in Iraq and Afghanistan, as well as small arm attacks of the kind used to devastating effect in the 2008 Mumbai rampage that killed 166 people in India.

The list of potential targets now includes hotels, sports stadiums and other public areas as well as famous landmarks, airliners, chemical plants and ports.

"Unlike large-scale, coordinated, catastrophic attacks, executing smaller-scale attacks requires less planning and fewer pre-operational steps. Accordingly, there are fewer opportunities to detect such an attack before it occurs," Napolitano said.

The threat of home-grown terrorism has also taken on a higher profile with the March indictment of Colleen LaRose, a blond Pennsylvania woman known as "Jihad Jane" who is accused of plotting over the Internet to kill a Swedish cartoonist for depicting the Prophet Mohammed in a manner offensive to Muslims.

About two dozen Americans have been arrested on terrorism charges since 2009, according to Napolitano.

"While it is not clear if this represents an actual increase in violent radicalization ... it is nonetheless evident that over the past 12 months efforts by violent extremist groups and movements to communicate with and recruit individuals within the United States have intensified," she said in her testimony. (Additional reporting by Jeremy Pelofsky; Editing by Anthony Boadle)