In his weekly radio and Internet address, the president asked Americans to take time "for prayer and reflection -- for the victims of this terrible tragedy, for the people who knew them and loved them, for those who are still struggling to recover, and for all the victims of the less publicized acts of violence that plague our communities on a daily basis."
At least 12 people were killed and 59 others were wounded in the rampage in an Aurora, Colo., movie theater where "The Dark Knight Rises" was opening in a midnight screening Friday morning. One suspect, James Eagan Holmes, 24, was in custody and police said they were "confident he acted alone."
In remarks that largely echoed what he said Friday as news of the massacre spread throughout the world, Obama said his administration "will take every step possible to ensure the safety of all our people."
"And the federal government stands ready to do everything necessary to bring whoever's responsible for this heinous crime to justice," he said.
"And we will stand by our neighbors in Colorado during this extraordinarily difficult time."
The president said it may never be known "what leads anyone to terrorize their fellow human beings."
"Such evil is senseless -- beyond reason," he said. "But while we will never know fully what causes someone to take the life of another, we do know what makes that life worth living.
"And if there's anything to take away from this tragedy, it's a reminder that life is fragile," the president said. "Our time here is limited and it is precious. And what matters in the end are not the small and trivial things which often consume our lives. It's how we choose to treat one another, and love one another. It's what we do on a daily basis to give our lives meaning and to give our lives purpose."