US President Barack Obama on Tuesday played down prospects for a spectacular Al-Qaeda attack 10 years after the September 11th strikes, saying he worried more now about solitary extremists. "The most likely scenario that we have to guard against right now ends up being more of a lone wolf operation than a large, well coordinated terrorist attack," he told CNN television during a campaign-style swing through Iowa. "We still have to stay on top of it, though. We're never letting our guard now, that's part of our job," said Obama, who vowed "heightened" security measures and "extra vigilance" ahead of...