U.S. President Barack Obama and his Republican challenger have suspended campaigning out of respect for victims of the shooting rampage at a Colorado movie theater.

Twelve people were killed when the gunman opened fire Friday during the midnight premiere of the latest Batman movie in the Denver suburb of Aurora. Seventy others were wounded, some seriously.

Mr. Obama called off a re-election rally and ordered flags to be flown at half-staff. Republican Mitt Romney also suspended campaigning, calling the incident “a hateful act.”

The shooter, arrested a short time after the rampage, was identified as 24-year-old James Holmes. The motive for the attack is not clear. Police say there is no indication of a link to any terrorist group.

The movie theater attack was the worst mass shooting in the United States since 2009 when an Army psychiatrist killed 13 soldiers and civilians at a military base in Texas.