WASHINGTON — Efforts by terrorists abroad to radicalize and recruit U.S. residents present new security threats, three top Obama administration officials told Congress on Wednesday.

The threat posed by homegrown extremists shows that the battle against terrorism has become more complex in the past year, underscoring the challenges of pinpointing and blocking plots, said Michael Leiter, director of the National Counterterrorism Center.

“Groups affiliated with al-Qaida are now actively targeting the United States and looking to use Americans or Westerners who are able to remain undetected by heightened security measures,” FBI Director Robert Mueller told the Senate Homeland Security and Governmental Affairs Committee.

It appears that “domestic radicalization and homegrown extremism” is becoming more pronounced, Mueller said.

Homeland Security Secretary Janet Napolitano said al-Qaida has inspired an array of terrorist organizations.

“We are all seeing more diverse activity” by a more diverse collection of groups, Napolitano said.

Leiter said al-Qaida in Pakistan is at one of its weakest points organizationally. Nonetheless, he said, the terrorist group remains a capable and determined enemy that has proven its resilience over time.

Since 2009, at least 63 American citizens have been charged or convicted for terrorism or related crimes, “an astoundingly high number of American citizens who have attacked — or intended to attack — their own country,” said Sen. Joe Lieberman, I-Conn., the committee’s chairman.

A year ago, the FBI arrested Michael C. Finton in Illinois and Hosam Smadi in Texas in connection with unrelated bomb attempts. The bureau used online undercover agents and confidential human sources who monitored Finton and Smadi until their arrests.

Several U.S. residents from Somali-American communities in Minneapolis were recruited to fight with the Somali-based terrorist group al-Shabab. That prompted the FBI to deploy bureau personnel to cities with high ethnic Somali populations in an outreach initiative to community leaders.

In his prepared testimony, Mueller said it is possible that more American extremists are feeling increasingly disenchanted with living in the United States or are angry about U.S. and Western foreign policy, “making their decision to leave for extremist opportunities abroad all the more appealing.”

Omar Hammami, an Alabama man now known as Abu Mansour al-Amriki, or “the American,” has become one of al-Shabab’s most high-profile members and appeared in a jihadist video in May 2009.

Leiter said the rising profiles of U.S. citizens like Hammami in overseas terrorist groups provide young extremists with American faces as role models.

Leiter said plots by homegrown Sunni extremists were disrupted in New York, North Carolina, Arkansas, Alaska, Texas and Illinois in the past year and point to “a collective subculture and a common cause,” even though the plots were unrelated.

Napolitano said U.S.-born, Yemen-based Anwar al-Awlaki is an illustration of an English-speaker spreading propaganda over the Internet, an approach she said could be helping to increase the number of homegrown extremists.

Terrorists “are working increasingly to build alliances or essentially recruit soldiers for their army from within the United States,” Lieberman said.

The panel’s ranking Republican, Sen. Susan Collins of Maine, said the shootings at Fort Hood, Texas, and the attempted Christmas Day attack on a Detroit-bound airliner show that the terrorist threat “is evolving and ever-changing” and “a chameleon by design.”