University of Texas at Austin student Elissa Garza said she and her friend Ileanna Guerri were having lunch, enjoying "a beautiful day" on campus Monday, when someone attacked nearby students with a knife, killing one and injuring three others.

"It didn't seem real," Garza said in an interview with ABC News on Monday. "I had never seen so many people's faces in utter terror."

Garza, one of at least 25 students who witnessed the attack, outside the Gregory Gym, said school officials urged people to go on with their normal schedules until they could gather more information about the situation. She went to her class, next door to the gym, but found it hard to focus on her work, she said.

"I was a little terrified of being in that building, but my teacher was extremely understanding. She let us turn in our assignment and walk out," Garza said. "She knew it was a little uneasy on campus already."

1 dead, 3 injured in UT-Austin stabbing; suspect in custody: Police

1 dead, 3 injured in University of Texas stabbing

Authorities are working to determine a motive behind the attack.

The university said it apprehended 21-year-old UT-Austin student Kendrex White, who allegedly attacked the students with what appeared to be a large hunting knife and "calmly" walked away, according to police.

He is being interrogated by police and has not yet been charged.

One of the victims died at the scene, according to UT-Austin police.

Two of the three other victims were released from the hospital on Monday, according to a university statement late that day. The university didn't provide details on the third surviving victim in the statement.

Guerri said that she has been thinking about the stabbings a lot since that afternoon and that she fears another similar attack could take place.

"It was the middle of the day, and this happened ... He was a student," she said in an interview with ABC News on Monday. "It's hard for me to not worry about that, to not worry that that's going to happen again."

Other students have expressed similar fears.

"A lot of people have been saying they don't want to go outside or go back to their dorm. They don't want to walk to classes tomorrow," according to first-year UT-Austin student Rachel Prichett, who said she was standing in line at a food truck near the gym when she heard screaming.

She said she was just a few feet from a student when he was stabbed.

"It is pretty terrifying," Prichett said. "If I had been facing the other away, had my back to that guy, it could have been me that he targeted."

In a letter released late Monday, UT-Austin President Gregory Fenves acknowledged that the campus has been gripped by "very understandable" fear since the attack.

"There is a great deal of fear on campus and in our community,” he said. "That fear is very understandable, especially in light of recent vandalism and threats against students."

Tomorrow will be a very difficult day for everyone on the Forty Acres, so please, be there for one another: https://t.co/pQfvuGCctc pic.twitter.com/H31JZBSX0Z — Greg Fenves (@gregfenves) May 2, 2017

Fenves did not offer details on vandalism or threats mentioned in the letter. Prichett told ABC News that she was aware of "a couple" of alleged threats on campus.

"There have actually been a couple of other supposed threats that have happened," she said. "There was a banner up somewhere that said, 'Tuition pays for bombing' or something like that."

Fenves wrote that classes and events will resume on Tuesday and that the school plans to hold a "gathering to honor the victims ... in the coming days."

"And I know that it will be a very difficult day for everyone on the Forty Acres, so please, be there for one another," he continued, using a nickname for the campus. "Please keep your thoughts and prayers with the victims and their families during this very difficult time."

ABC News' Kevin Pliszak, Bonnie Mclean and Becky Perlow contributed to this report.