She didn't see the gunman or hear the shots but knew what was happening.

As a young man carried out a deadly shooting Thursday at Florida State University, Stephanie Horowitz looked out at the sprawling campus and saw a dreadful reminder that brought her back to when she was a teenager at Marjory Stoneman Douglas High School during the Parkland massacre seven years ago.

“You could almost see the silence. There was not a soul in sight and belongings left behind like open laptops and bags," Horowitz said in an interview with The Associated Press.

"I knew what that meant, because I've done this before. I know what the aftermath of a school shooting looks like.”

Horowitz, a graduate student at Florida State University, is among a small group who were in the traumatising midst of both the massacre in Parkland and now the shooting at the college in Tallahassee, inexplicably forced to endure a second school shooting in the early stages of their adult lives.

“You never think it's going to happen to you the first time, you certainly never think it's going to happen to you twice,” said Horowitz, 22. “This is America.”

Two people were killed and six others were injured after a 20-year-old man, identified by police as Phoenix Ikner, opened fire around lunchtime Thursday near a student union building on the Florida State University campus.

The suspect, a student at the university and the stepson of a sheriff's deputy, was hospitalized with injuries that are not considered life-threatening, police say.

Florida State student Logan Rubenstein was in eighth grade when he was forced to shelter in place at his middle school during the Marjory Stoneman Douglas High School massacre nearby.

"What we went through, we made it our mission to ensure this could never happen again,” said Rubenstein, 21. “And I'm sorry that we weren't good enough because now this is the second shooting that I've had to go through.”

The Marjory Stoneman Douglas High School shooting was one of the deadliest school shootings in US history, with 17 people killed and 17 others wounded on Valentine's Day in 2018.

Jaclyn Schildkraut, who leads a gun violence research group at the Rockefeller Institute of Government in New York, said that experiencing multiple school shootings could prolong a person's emotional healing process.

“It's like all of that progress that you've made seemingly goes away and you're right back at the starting line,” she said.

Lori Alhadeff, whose daughter, Alyssa, was killed in the Parkland shooting, said she felt a wave of panic wash over her when her son Robbie texted her that there was an active shooter at Florida State, where he is a student.

“It's never the message that you want to get, that there's a shooter at your child's school,” Alhadeff said. “Your brain just really starts to spin, and it's traumatizing and obviously very triggering to me and my husband and my son.”

She said her son was in the student union about 20 minutes before the shooting but left before the gunman arrived.

“I pray for the families that lost somebody yesterday, but this should not be normal,” said Alhadeff. “This should have not been my son's second experience with a school shooting. We need to do better.”