PROVIDENCE, R.I. (WPRI) — “Really simple. Go with your gut feeling.”

That’s what Brown University custodian Derek Lisi is saying after he noticed and reported a suspicious person on campus numerous times before last weekend’s deadly shooting.

Lisi told NewsNation affiliate WPRI he remembers seeing the gunman more than a dozen times, canvassing the Barus and Holley building and surrounding areas before the attack Dec. 13.

“We interact with the students all the time,” Lisi said. “This hits close to home because it’s our workplace. We spend more time there than we do at home because of the shift that we work.”

Lisi works at the Engineering Research Center, which is a short walk from Barus and Holley, and often goes through the latter on his way to take out the trash.

He recalled seeing the shooter — later identified as Claudio Manuel Neves Valente — at about 6:20 p.m. the day before Thanksgiving and then just after 8 p.m. on Dec. 1.

“Something kept telling me, ‘Don’t ignore it. Don’t ignore it,'” he said.

Lisi said he reported the individual as suspicious to Event Staff Services LLC, or ESS — a third-party security vendor for Brown — on three separate occasions.

When he warned them Dec. 1, a staff member of that security company told him they were only present to cover an event. Lisi said the university’s Department of Public Safety might have been notified, but he doesn’t know if any other action happened.

ESS did not respond to multiple requests for comment. According to the Boston Globe, David Madonna, the company president, said investigating reports of suspicious people is not its job.

“We have nothing to do with watching buildings,” Madonna told the Globe. “Whenever there’s an event at Brown, they hire us to do ID check and capacity counts in their rooms.”

A spokesperson for Brown University did not respond to multiple requests for comment. The school has hired former Rhode Island U.S. attorney Zachary Cunha as outside counsel, WPRI confirmed.

After police released photos of the suspect captured on surveillance video, Lisi said he “dropped” because he immediately recognized the man he’d been hoping wasn’t the suspect. He immediately called the tip line, which led to him later meeting with detectives Dec. 15 to describe what he witnessed.

“I told detectives, it was like I could see him and nobody else could — it is an eerie feeling,” Lisi explained. “It was the way he was browsing in the classrooms. He was like rolling his eyes one way, rolling his eyes another way. And it wasn’t just that classroom, it was all the classrooms.”

According to a police affidavit released publicly, Lisi told officers he recognized Neves Valente by his unusual gait, which he described as a limp. He was wearing the surgical mask and dressed in the same clothing as the man in the video, according to the affidavit.

Lisi told WPRI that no one he had encountered on campus had ever made him feel that way.

Police said Neves Valente traveled to Brookline, Massachusetts, two days after the Brown shooting and murdered MIT professor Nuno Loureiro. He then traveled to a storage facility in Salem, New Hampshire, where police found him dead Thursday, Dec. 18.

A New Hampshire federal medical examiner said he died from a gunshot wound to the head. The cause of death was suicide, according to the report. Federal investigators later linked ballistics from both attacks to two pistols recovered from the storage facility.

Lisi said the “what ifs” continue to swirl in his mind.

“Yes, the shooter is responsible,” he said. “But security should have been better, without a doubt.”

Lisi told WPRI that the doors of Barus and Holley are usually locked on the weekends, but that wasn’t the case the day of the attack.

“That the building should have been open access,” he said. “That’s what is still weird to me. It doesn’t make sense to me why on a Saturday it was an open access.”

If there were more cameras in the building, Lisi wonders if authorities might have been able to identify Neves Valente sooner and potentially prevent another tragedy.

“Two students aren’t here today — that kills me every day,” Lisi said. “I can’t get his face out of my head.”

However, Lisi said he doesn’t blame anyone but the shooter for the tragedy, because no one ever expected or imagined something like this could happen on Brown’s campus. Having worked at the university for nearly 15 years, Lisi described the environment as a tight-knit community, adding that he has a close relationship with many students.

“I always look out for them,” he said. “I always interact with them when I’m cleaning. I see the same kids every day. I know the personalities. I know who’s having a good day and who isn’t. They’re very comforting, those kids.”

Lisi also called on Brown to install a permanent memorial for Ella Cook and MukhammadAziz Umurzokov, who were both murdered.

“Students should never forget,” he said. “Nobody should forget what happened.”

When it comes to staying vigilant, Lisi urged people to trust their instincts and not to be afraid to speak up if something seems off. Lisi said he doesn’t know the unidentified tipster police refer to as “John,” who is a Brown graduate currently experiencing homelessness.

He lauded the tipster for doing the right thing and coming forward. R.I. Attorney General Peter Neronha said the information he provided “blew the case wide open.”

Lisi said part of the reason he decided to speak out was to provide some clarity for people who were mistaking him for John, and avoid confusion. He encouraged people to be mindful and respectful to all the students and families who are suffering.

“If you see something’s off, call it in right away,” he said.