Grieving students at the University of North Carolina at Charlotte left a memorial of flowers, candles and signs outside the Kennedy building Thursday after a gunman opened fire in a classroom there.
Trystan Andrew Terrell is accused of gunning down two students and injuring four others in a classroom on Tuesday, according to police.
"I think my heart stopped when I got the notification" of an active shooter, Kelly Redenbaugh, whose son is a freshman, told ABC News. "Once I knew he was inside, safe, I could breathe again."
"It's tough to think we send our kids off to school, we expect them to come back," said UNC Charlotte alumnus Jim Heffner. "You hear about it on the news all the time... but then when it hits right here at your home, it's a different story. It changes everything."
Terrell's choice of building appeared intentional, said Charlotte-Mecklenburg Police Chief Kerr Putney, but a motive remains unclear.
"It's going to take us some time to really figure out what happened, and more importantly, the why," Putney said.
Terrell waived his right to appear at his first court appearance Thursday afternoon. He was appointed an attorney and a bond hearing was set for May 15.
Faculty and students gathered at a vigil Wednesday night in an arena on campus to remember those killed: 19-year-old Ellis "Reed" Parlier and 21-year-old Riley Howell.
"We're heartsick," UNC Charlotte Chancellor Philip Dubois said at the vigil. "The loss of Riley and Reed cuts to our core."
"No one is going through this alone," said student body president Chandler Crean as he wiped away tears.
The four injured students are expected to make full recoveries.
Among the injured is 23-year-old Emily Houpt, who is set to graduate next week.
Howell was killed while fighting the gunman, said Putney.
Putney called Howell a "hero" whose "sacrifice saved lives."
Terrell has been charged with two counts of murder, four counts of attempted murder, four counts of assault with a deadly weapon with intent to kill, possession of a firearm on educational property and discharging a firearm on educational property.
ABC News' Steve Osunsami and Janice McDonald contributed to this report.