US President Barack Obama called the shootings at a movie theater in Colorado on Friday a reminder that life is fragile and promised that the federal government stood ready to do all it could to seek justice for the "heinous crime."
Obama cut short a campaign trip in Florida on Friday to return to Washington after a gunman killed 12 people and wounded 59 at a midnight screening of the new Batman movie in Aurora, Colorado, a Denver suburb.
"Even as we come to learn how this happened and who's responsible, we may never understand what leads anyone to terrorize their fellow human beings," Obama said in his weekly radio and Internet address, which was broadcast on Saturday.
"Such evil is senseless - beyond reason," he said.
Police took James Eagan Holmes, 24, into custody on Friday in a parking lot behind the cinema.
He is expected to make his first court appearance on Monday.
The shooting evoked memories of the 1999 massacre at Columbine High School in Littleton, also a Denver suburb.
Obama said "the federal government stands ready to do everything necessary to bring whoever's responsible for this heinous crime to justice."
"And if there's anything to take away from this tragedy, it's a reminder that life is fragile. Our time here is limited and it is precious," the president said in his address.
Speaker of the House of Representatives John Boehner echoed the president's sentiment in the weekly Republican address.
"I know that when confronted with evil we cannot comprehend, Americans pull together and embrace our national family more tightly," Boehner said.
"We join President Obama in sending condolences and prayers to the loved ones of those who were killed and wounded. And we all say 'thank God' for the police, the first responders, the doctors, and the nurses whose swift and heroic efforts saved lives."