For years after the 9/11 attacks, Americans fretted about fundamentalist Islamic cells infiltrating the homeland and jihadists hiding in their midst.
The latest mass shooting at a US military base - and the second in less than five years at Fort Hood, Texas - raises a potentially more disturbing question: have the Iraq and Afghanistan wars created American-grown human time bombs with grievous mental and physical wounds that the military and veterans' health care systems can't adequately track and treat?
Senior army officials provided conflicting views of Specialist Ivan Lopez's state of mind before his gun rampage at Fort Hood on Wednesday.
Lopez killed three people at the base and wounded 16 others before taking his own life.
Lopez bought the gun he used in the attack on March 1, but had not registered it at Fort Hood as required by regulations.
"We have strong evidence he had a medical history that indicates an unstable psychiatric or psychological condition," said Lieutenant General Mark Milley, the commander at Fort Hood.
"We believe that is the fundamental underlying causal factor."
But army secretary John McHugh told Congress that a psychiatrist's examination of Puerto Rico-born Lopez last month had not disclosed deep enough problems for him to be considered a threat to others.
"As of this morning, we had no indication … that there was any sign of likely violence, either to himself or to others," McHugh told the Senate Armed Services Committee.
Lopez, who served four months in Iraq, "was undergoing a variety of treatment and diagnoses for mental health conditions, ranging from depression to anxiety to some sleep disturbance", McHugh said. Relatives of Lopez said he told them he sustained a brain injury and was suffering from post-traumatic stress disorder, or PTSD.
Whatever Lopez's condition or motive turns out to be - should it become known at all - his rampage brings to at least nine the number of fatal criminal shooting incidents at US military bases since 2008.
In November 2009 at Fort Hood, Major Nidal Malik Hasan killed 13 people and wounded 32 others. Hasan, a self-avowed "soldier of Allah", was sentenced to death last year and is in jail at Fort Leavenworth, Kansas.
Aaron Alexis, a former navy sailor, shot 12 people dead last August at the Washington Navy Yard before he was killed.
Harry Croft, a former army psychiatrist who has treated 1,700 cases of PTSD, said he and other health care providers tread a thin line between trying not to exaggerate such problems and imploring the government to do more to address them.
Croft said it was very difficult to identify such insider threats in advance. "We still can't predict with tremendous accuracy whether someone is going to go out and hurt themselves or other people," Croft said.
"I wish we could, but it's as much an art as a science."