US President Barack Obama cut short a re-election campaign trip to Florida on Friday following a mass shooting at a Batman movie that he branded as "senseless" and "evil."
Obama told a somber crowd at a campaign rally in Fort Myers that the shooting in the Colorado town of Aurora which left at least 12 people dead "reminds us of all the ways that we are united as one American family."
"Such violence, such evil, is senseless. But while we will never know fully what causes somebody to take the life of another, we do know what makes life worth living," Obama said.
"The people we lost in Aurora loved and they were loved," he said.
At least 12 people were killed and around 40 others wounded in the rampage early Friday at a movie theater in Aurora, a town outside Denver, showing the latest Batman film.
Obama, who cancelled a planned election campaign stop in Orlando to return to Washington, earlier released a statement saying he and his wife, Michelle, were "shocked and saddened" by the slayings.
"My administration will do everything that we can to support the people of Aurora in this extraordinarily difficult time," he said.
"We are committed to bringing whoever was responsible to justice, ensuring the safety of our people, and caring for those who have been wounded.
"As we do when confronted by moments of darkness and challenge, we must now come together as one American family," the president said.
Obama was notified of the mass shooting at 5:26 am (0926 GMT) by his homeland security advisor John Brennan, according to White House spokesman Jay Carney, and spoke with Federal Bureau of Investigations chief Robert Mueller.
Carney said the shooting did not appear to be terrorist-related.
US television networks have identified the gunman as James Holmes, a 24-year-old Aurora man.
"This is a local law enforcement investigation at this point, and what we can say is that we do not believe at this point that there is an apparent nexus to terrorism," Carney said.
Carney said federal agents with the FBI and bureau of Alcohol, Tobacco and Firearms (ATF) had arrived to help local officials in Colorado.
Mitt Romney, Obama's Republican rival in the November 6 presidential election, and his wife, Ann, also expressed shock at the slayings.
"We are praying for the families and loved ones of the victims during this time of deep shock and immense grief," they said in a statement.
"We expect that the person responsible for this terrible crime will be quickly brought to justice."
Witnesses described a scene of chaos as the gunman burst into a packed showing of "The Dark Knight Rises," the latest Batman film, setting off a smoke device of some kind and spraying the theater with bullets.
Police have detained the gunman but have released few details about his identity or motives.