The former University of Arizona student who allegedly gunned down 52-year-old professor Thomas Meixner on campus Wednesday afternoon was in the process of being banned from campus, officials said.

Meixner, the head of the university’s Department of Hydrology and Atmospheric Sciences, was shot by Murad Dervish inside the John W. Harshbarger Building at the university’s main campus in Tucson at about 2 p.m., according to officials.

Dervish, 46, was not currently enrolled at the university and was set to be legally barred from the campus for unknown reasons, University of Arizona Police Department Sgt. Sean Shields told The Post Thursday.

Officers had been called to escort the suspect out of the building when multiple shots rang out, police said. Meixner was later pronounced dead at a local hospital.

“He was going to be issued an exclusionary order … it wasn’t done then but it was going to be done in a couple days,” Shields explained.

“Our exclusionary orders are basically if, you know, a university official asks for someone to be excluded, we can exclude them from a certain building. Or if like, someone is using the university for unintended purposes, like if they’re sleeping or if they’re looking up things on the main library’s websites on things they shouldn’t be looking up,” he said.

“Then if they get on the exclusionary order list and then if we come into contact with them, they could be subject to arrest for trespassing.”

The murder suspect was tracked down by state police later that evening in Gila Bend, Arizona, more than 100 miles away, police said. He was brought back to Tucson by university police Thursday morning and was being held in the Pima County Jail without bond, according to Shields and public records.

Dervish had been previously known to university police, and officials were in the process of pulling his case reports, Shields said, without elaboration.

The shocking violence left the campus under lockdown as students and teachers grieved the loss of the esteemed educator.

“We have lost a beloved member of our University of Arizona community,” university president Robert C. Robbins said in a statement.

“This incident is a deep shock to our community, and it is a tragedy. I have no words that can undo it, but I grieve with you for the loss, and I am pained especially for Tom’s family members, colleagues and students.”

Meixner had received his PhD at the university in 1999 and was an expert on watershed hydrology, atmospheric chemistry and climate change impacts on water resources, according to the college.

Classes had resumed Thursday and counseling services were being provided to students, the university said.