Nov. 2 (UPI) -- Colorado authorities said Thursday they've arrested a man they believe shot three people dead at a Denver-area Walmart.
Authorities had earlier opened a large manhunt for Ostrem, 47, with nationwide extradition warrant.
Thornton, Colo., police said the gunman started randomly shooting inside the Walmart store Wednesday evening. Three people were killed.
"This is a very heinous act," police spokesman Victor Avila said. "We don't know exactly what the motive of the person was, but it was certainly a terrible act."
Two men died at the store and a woman died at a hospital. They were not immediately identified. No one else was injured.
"He walked in very nonchalantly with his hands in the pockets, raised a weapon and began shooting," Avila added.
"It's a crazy world we live in."
Investigators, including special agents of the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives and FBI, are involved in the case.
"Here's Las Vegas happening in Colorado -- at that moment all I could think about was my life," witness Aaron Stephens said, referring to the mass shooting in Las Vegas last month. "At first I thought it was a firecracker."
Thornton is located about 10 miles north of Denver.