Law enforcement officials say they believe the gunman who opened fire with an assault rifle at a University of Texas campus library before killing himself was a student who acted alone.

Campus police Chief Robert Dahlstrom said at a Tuesday news conference that officials have ruled out the possibility of a second shooter. The initial confusion was because the suspect fired shots in multiple locations.

Austin police have identified the shooter as 19-year-old student Colton Tooley.

Officials said a man wearing a ski mask entered the Perry-Castenada Library at the University of Texas at Austin close to 8 a.m. on Tuesday and fired an AK-47 assault rifle before killing himself.

Up to 10 shots were fired, witnesses told the Dallas Morning News, and the gunman appeared to be smiling as he made his way through the library. 

Rhonda Weldon, the university's director of communications, confirmed that police later found a body inside the library that is believed to be the gunman. 

Authorities said no one else was injured and that investigators are trying to determine what led to the gunfire.

Police had systematically searched every classroom for another possible suspect on the sprawling campus but determined that the gunman acted alone.

By the afternoon, the university had issued an all clear for faculty and students.  

Randall Wilhite, an adjunct law professor at the university, said he was driving to class when he saw "students start scrambling behind wastebaskets, trees and monuments," and then a young man carrying an assault rifle sprinting along the street.

"He was running right in front of me ... and he shot what I thought were three more shots ... not at me. In my direction, but not at me, clearly not at me," Wilhite said.

The professor said the gunman had the opportunity to shoot several students and Wilhite, but he did not.

Classes remained cancelled Tuesday at the 50,000-student campus -- the site of one of the nation's deadliest shooting rampages four decades ago. 

On Aug. 1, 1966, Charles Whitman went to the 28th floor observation deck at the UT clock tower and began shooting at people below. He killed 16 people and wounded nearly three dozen before police killed him about 90 minutes after the siege began.

Click here to read more on this story from the MyFoxAustin.com 

Click here to read more on this story from the Dallas Morning News 

The Associated Press contributed to this report