A student wearing a dark suit and a ski mask opened fire Tuesday morning with an assault rifle on the University of Texas campus before fleeing into a library and fatally shooting himself.
No one else was hurt in the shooting at the university, which remained closed Tuesday night.
Authorities said 19-year-old Colton Tooley, a sophomore math major, walked through the heart of the campus, firing. The shooting began near a fountain in front of the UT Tower -- the site of one of the nation's deadliest shooting rampages more than four decades ago, when a gunman ascended the clock tower and fired down on dozens of people.
Police declined to speculate on Tooley's motive.
Before reaching the library, Tooley apparently walked for several blocks wearing a mask and dark clothing and carrying an automatic weapon, witnesses said.
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"At least he didn't kill anyone else," student Caren Garcia said. "It was just a crazy, crazy morning."
Student Eduardo Aramburo was one of the first people to see him running into the library. He said he could hear bullets flying through the air.
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"When you experience it, it's like another emotion, like nothing else -- it's like you're trying to save your life," he said.
Police said it was unclear whether Tooley was targeting anyone with the AK-47. Witnesses said the gunman could have fired at dozens of students, but didn't.
After the gunfire, authorities searched the campus for a possible second shooter, but eventually concluded Tooley acted alone. Confusion about the number of gunmen arose because shots were fired in multiple locations, and officers received varying descriptions from witnesses, campus police Chief Robert Dahlstrom said.
Acevedo said officers were able to track the gunman's movements with the help of students who "kept pointing in the right direction."
The police chief said he believes Tooley ran into the library as officers closed in on him, then shot himself in the head on the sixth floor. Police did not fire any shots, Acevedo said.
Tooley lived with his parents in Austin, who did not immediately respond to a message left by The Associated Press.
A man who said he was a relative of the family and would identify himself only as Marcus came out of their home late Tuesday and said Tooley's parents were distraught over losing their child.
"I want you to understand how he lived. He was a very smart guy, very intelligent, excellent student. He wouldn't or couldn't hurt a fly," he said, reading from a prepared statement. "This is a great shock to me and my family. There was nothing prior to this day, nothing that would lead any of us to believe this could take place."
Tooley's high school principal in Austin described him as an excellent student who excelled in every subject.
"All of us in the Crockett High School community are shocked and saddened by today's tragedy at the University of Texas," said principal Craig Shapiro.
His prepared statement said teachers remembered Tooley as being "brilliant," "meticulous" and "respectful." The Austin Independent School District said Tooley, a 2009 graduate, excelled in every subject and was ranked seventh in his class.
Police investigators went in and out of his family's home in a middle-class Austin neighborhood Tuesday afternoon carrying bags and boxes. There was no immediate word on what was in the containers. A neighbor said police arrived at the home about three hours after the campus shooting.
UT President Bill Powers credited the school's crisis-management plan and social networking for quickly warning students, faculty and staff. The university's text messaging system reaches more than 43,000 people, he said.
The university has about 50,000 students and 21,000 faculty and staff. It has more than 200 buildings that sit on 350 acres and is one of the largest universities in the country.
On Aug. 1, 1966, Charles Whitman went to the 28th floor observation deck at the UT clock tower in the middle of campus 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.
NBCDFW's Grant Stinchfield contributed to this report.
