CHAPEL HILL (AP) — A University of North Carolina graduate student walked into a classroom building, shot his faculty adviser and quickly left, authorities said a day after the attack paralyzed campus as police searched for the gunman.

Tailei Qi, 34, was charged Tuesday with first-degree murder and having a gun on educational property in Monday’s killing of Zijie Yan inside a science building at the state’s flagship public university.

Chapel Hill police arrested Qi without force in a residential neighborhood near campus within two hours of the attack, UNC Police Chief Brian James said at a news conference.

Investigators were trying to determine a motive and searching for the gun, James said. He declined to specify where in Caudill Labs Yan was killed, saying officers are still looking at evidence. Qi was already gone when a team of officers reached the building, James said.

Yan was “a beloved colleague, mentor and a friend of so many on our campus and a father to two young children,” UNC Chancellor Kevin Guskiewicz at the news conference.

On Wednesday afternoon, the school’s iconic Bell Tower will ring in honor of Yan’s memory and students are encouraged to take a moment of silence, he said. The school also canceled classes until Thursday.

Earlier Tuesday, Qi briefly appeared in Orange County Superior Court in Hillsborough. Judge Sherri Murrell ordered Qi to remain jailed without bond and scheduled his next court date for Sept. 18. After the hearing, Qi bowed to the judge, his Mandarin interpreter, public defender Dana Graves and the guards who took him away in handcuffs.

Graves left court without talking to reporters and did not immediately respond to an email seeking comment.

Yan was an associate professor in the Department of Applied Physical Sciences who had worked for the university since July 2019, Guskiewicz said Tuesday. He led the Yan Research Group, which Qi joined last year, according to the group’s UNC webpage.

Yan was a respected and approachable professor and research adviser who was deeply knowledgeable about the field, said Wen Liu, a 2022 graduate who worked in the lab for three years.

He was somewhat reserved, yet always willing to answer questions with patience and respect and advise lab members who got stuck in their research, Liu said.

“For hours he would just be doing things and explaining along the way,” said Liu, who was a “newbie undergrad in the field” at the time and also worked with Qi in the lab. Qi seemed passionate about research, curious about others’ work and “pretty sociable,” Liu said.

The lab’s main goals were making and studying nanoparticles under the effect of light, using lasers, he said. The work has potential applications in medicine and other fields.

A since-deleted page on the school’s website listed Qi as a graduate student in Yan’s research group, with Yan as his adviser, though the police chief said their ties were still under investigation. Qi previously studied at Wuhan University in China before earning a masters in mechanical engineering at Louisiana State University in 2021.

The attack and hourslong lockdown terrified students and faculty who had returned last week for the start of the fall semester. On Tuesday, students pet therapy dogs on campus and chalked hearts, peace signs and messages of hope on walking paths.

Noel Harris, a senior journalism student, said she spent confusing and scary hours locked in a class reading news coverage, listening to police scanners and waiting for university updates about whether the danger had passed.

When an officer arrived, the class asked him to slide his badge under the door first, Harris said. The officer said they were safe but recommended they wait until an all-clear was issued. Soon after, Harris recorded video of people climbing out of an adjacent building’s windows, and she started to wonder “so is it really safe? What’s going on?” she said.

She said Tuesday that she was still trying to understand why the students left through the windows of Phillips Hall, where math and other classes are held but no shots were fired.

“I felt myself just being scared and shocked, but then not shocked at the same time because it’s like, this happens every day,” Harris said.