Army Chief of Staff George Casey asked Army leaders across the country Friday to review force protection measures after the shooting rampage at Fort Hood left 13 people dead.

Casey described the shooting as a "kick in the gut," not just for Fort Hood but for the entire Army.

He told reporters at a news conference that he was encouraged by the stories of heroism and courage that have emerged the day after the attack.

He cited a young private who was sitting in his pickup in a parking lot and heard gunfire. Casey said the private went to the scene, pulled out four of the wounded and drove to the hospital.

Moment of silence
Military, friends and families observed a moment of silence at U.S. military bases around the world as a show of respect for the victims. Dozens of people gathered at Fort Hood and bowed their heads as part of the ceremony.

Earlier in the day, President Barack Obama said the "entire nation is grieving" over the mass killings. He urged people not to jump to conclusions while law enforcement officials gather facts about the shootings.

Obama met Friday morning with FBI Director Robert Mueller and other federal leaders to get an update on what they've learned.

"We don't know all the answers yet. And I would caution against jumping to conclusions until we have all the facts," Obama said in a Rose Garden statement otherwise devoted to the economy.

"What we do know is that there are families, friends and an entire nation grieving right now for the valiant men and women who came under attack yesterday," the president said.