Christian leaders are calling for prayer after a gunman stormed a student night at a bar in Thousand Oaks, California, on Wednesday night, killing 12 people before turning the gun on himself.
The gunman has been identified in media reports as Ian Long, 28, a former Marine who previously served in Afghanistan.
Officials said Long carried out the attack in the Borderline Bar and Grill using a legally owned .45 caliber handgun.
In April this year, police officers had been called to Long's home in Newbury Park after reports that he had been behaving erratically, but he was cleared by a mental health specialist.
The victims of Wednesday's attack included Sgt. Ron Helus, who had served the Ventura County Sheriff's Office for 29 years and was one of the first on the scene during the attack.
Pastor and evangelist Greg Laurie said the attack was 'pure evil' as he urged people to pray.
'According to the Bible, there really is a devil, there really is evil, and because of this people can be be motivated to do unimaginable things, like a man walking into a bar and indiscriminately shooting people,' he said.
'This is why America needs to pray. We need to pray for the protection of people wherever they are, synagogues, churches, malls, restaurants and wherever else they congregate.'
He praised Sgt Helus as a 'true hero' as he asked for prayers for all of the victims' families.
'Let's all continue to pray for the families of those who were killed. Their lives will be changed forever. They need to turn to the 'God of all comfort' in a time like,' he said.
Jentezen Franklin, senior pastor of Free Chapel in Gainesville, Georgia, said: 'Our thoughts and prayers go out to those fighting for their lives and the families of the victims of the senseless violence that took place last night in Thousand Oaks, California.
'There aren't laws that can stop a person committed to hurting other people, but it does add a sense of urgency for reaching all people with the message of hope and life change found in Jesus Christ.
'Please join me in praying for peace and safety in our nation and for these precious families who have lost so much in this tragedy.'