A student wearing a dark suit and a ski mask and armed with an assault rifle began shooting at the University of Texas campus this morning before fleeing into a library and shooting himself dead.
The shooting began near a fountain in front of the UT Tower - the site of one of America's deadliest shooting rampages more than 40 years ago, when a gunman climbed the clock tower and fired on dozens of people, killing 16.
Within hours of yesterday's gunfire, the school issued an all-clear notice, but the university remained closed and the area around the library was considered a crime scene.
There are not thought to be any deaths or injuries arising from the incident, other than that of the student himself.
Authorities identified the gunman as 19-year-old Colton Tooley, a sophomore maths student. Police would not speculate on his motive.
A man who said he was a relative of the family and identified himself only as Marcus came out of Tooley's parents' home early today and said they were distraught.
"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 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 Craig Shapiro.
Mr Shapiro said Tooley, a 2009 graduate, was remembered by teachers as being "brilliant", "meticulous" and "respectful".
Police went in and out of his family's home in a middle-class Austin neighbourhood last night, carrying bags and boxes. There was no immediate word on what was in the containers. A neighbour said police arrived at the home about three hours after the campus shooting.
The 50,000-student university had been on lockdown while officers with bomb-sniffing dogs carried out a building-by-building manhunt.
After the gunfire, authorities searched the campus for a possible second shooter, but eventually concluded Tooley acted alone. No-one was hurt during the rampage.
Confusion about the number of suspects arose because shots were fired in several locations and officers received varying descriptions from witnesses, campus police chief Robert Dahlstrom said.
Before reaching the library, Tooley apparently walked for several blocks wearing a mask and dark clothing and carrying an automatic weapon, witnesses said.
Building worker Ruben Cordoba said he was installing a fence on the roof of a three-storey building near the library when he looked down and made eye contact with Tooley.
"I saw in his eyes he didn't care," Mr Cordoba said.
The gunman continued down the street, firing three shots towards a campus church, then changed direction and fired three more times into the air, Mr Cordoba said.
A dustcart driver leapt out of his vehicle and ran away, as did a woman carrying two babies, Mr Cordoba said.
Randall Wilhite, an adjunct law professor, said he was driving to a 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," Prof Wilhite said.
The professor said the gunman had the opportunity to shoot several people, but did not.
Police said it was unclear whether Tooley was targeting anyone with the AK-47.
Police Chief Art Acevedo said officers were able to track the gunman's movements with the help of students who "kept pointing in the right direction".
He said he believed 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, he said.
School president Bill Powers praised the school's crisis-management plan and social networking for quickly warning students and staff. He said the university's text messaging system reached more than 43,000 people.
Laura Leskoven, a graduate student from Waco, said she was in a media management class when she received a text message from the university saying there was an armed person near the library.
For the next three and a half hours, she and about 30 classmates sat in a locked conference room trying to keep tab on events through Twitter, blogs and text messages.
The university was the scene of another gun rampage on August 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.