US President Donald Trump slammed the attack, saying that terrorists were "losers."
"So many young beautiful innocent people living and enjoying their lives murdered by evil losers in life. I won't call them monsters because they would like that term. They would think that's a great name. I will call them from now on losers because that's what they are," he said.
"This wicked ideology must be obliterated."
A US Department of Homeland Security statement said it was "closely monitoring" the situation.