Documentary chronicles America’s first mass shooting (VIDEO)

A documentary hitting theaters this month details what’s considered to be America’s first mass shooting.

“Tower” doesn’t focus on the killer, though. It explores what it was like on the ground during the rampage that saw 13 people killed and dozens more injured.

On Aug. 1, 1966, an engineering student and U.S. Marine sniper climbed a clock tower at the University of Texas and opened fire. Charles Whitman, then 25, had killed his wife and mother before he climbed the tower and started the 90-minute shooting spree. The carnage ended when police killed Whitman. One victim died a week after the shooting, and medical examiners attributed another death to Whitman in 2001, bringing the total number killed to 17.

Director Keith Maitland uses animation, news footage, radio clips and interviews with eight survivors to tell the story. He told CBS News it was difficult to answer questions about Whitman’s motivations.

“We would never know the answers to those questions,” said Maitland. “But what was answerable was what it was like to survive.”

The Wall Street Journal got exclusive access to a tense scene from the film. An officer who responded to the shooting recounts what he saw on that hot August day.

Whitman shot people from all four sides of the tower’s deck, and police estimate he had nearly 700 rounds of ammunition.

“These random public acts are the most terrifying because there’s nothing you can do to prevent them,” Maitland said. “There’s no amount of vigilance you can have with somebody, especially a long-range sniper.”