WASHINGTON — Efforts by terrorists abroad to radicalize and recruit U.S. residents are an evolving problem, 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 terrorist 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.