"You ask, 'Is it serious?' Yes, you bet your life. People don't do that unless it's a serious situation," Rumsfeld told a Pentagon briefing.
But Rumsfeld said Americans should not cower in fear.
"We shouldn't sit around, you know, hiding under chairs and hoping it will go away. We have to live our lives and that's what we'll all do, and that's precisely what the president suggested," Rumsfeld said.