Officials warn of more terrorist threats within U.S.

"Homegrown" terrorism is of growing concern, witnesses and lawmakers agree during a Senate hearing.

The heightened pace of attempts by terrorists to attack the United States is not a fluke and is likely to continue, senior Obama administration officials told a Senate panel Wednesday.

"We are all seeing increased activity by a more diverse set of groups and a more diverse set of threats," Homeland Security Secretary Janet Napolitano told the Senate Homeland Security and Governmental Affairs Committee.

Homeland Security Committee Chairman Joe Lieberman, I-Conn., and ranking member Susan Collins, R-Maine, who have been investigating the threat of "homegrown" terrorism, said there have been 63 U.S. citizens either charged or convicted of terrorism-related crimes in the United States since 2009.

"As we have made it more difficult for terrorists to come from abroad, we are seeing the escalation of a significant new threat that takes advantage of radicalized violent Islamist extremists within our borders," Collins said.

She said a Congressional Research Service report found that since May 2009, arrests were made in 19 plots by U.S. residents, compared to 21 plots from September 2001 to May 2009.

Napolitano and FBI Director Robert Mueller told the committee that they assumed that the recent increase is not an aberration. The issue even has its own abbreviation -- CVE, for countering violent extremism.

Michael Leiter, director of the National Counterterrorism Center, said the government did not believe the trend is going to decline anytime soon.

Napolitano said there had been an increase in the use of English-language Internet propaganda to help radicalize people in the United States. There has also been a rise in smaller, faster-developing plots using improvised explosives or small arms, she added.

But Napolitano said the U.S. government did not have "a complete understanding" of what would cause a U.S. citizen to become radicalized.

Mueller said the radicalization of U.S. citizens was related to what occurs in countries like Yemen, Somalia, Pakistan and Afghanistan. He said influences that lead U.S. citizens to become radicalized come from abroad.

But Collins expressed concern that no single U.S. agency is in charge of identifying and stopping the recruitment of U.S. citizens to carry out terrorist attacks.

On another subject during the hearing, tensions rose between Napolitano and Sen. John McCain, R-Ariz., over what was being done to secure the country's southwest border and arrest illegal immigrants.

McCain said residents and local law enforcement officers in Arizona had not seen improvements in border security. He said they believed it was getting worse.

Napolitano, a former governor of Arizona, fired back, citing statistics that she said showed that the government was doing more than ever to secure the border and arrest and deport dangerous illegal immigrants.

At one point during their exchange, Napolitano and McCain both cited sheriffs in Arizona who would come to Washington and testify to back up their respective positions. "Let's get them all up here," Napolitano said.