TAMPA, Fla. (AP) — Two 27-year-old University of South Florida doctoral students have been missing for a week, and authorities have asked for any assistance in locating them.

Both Zamil Limon and Nahida Bristy, a couple from Bangladesh, were last seen on April 16 at the campus in Tampa, Florida, according to a statement from the University of South Florida Police Department.

Limon, who was studying geography, environmental science and policy, was last seen at his home in a student apartment complex. Bristy, who was studying chemical engineering, was last seen an hour later at a campus science building.

A family friend contacted authorities last Friday after being unable to contact either one, USF police said.

USF police investigators don’t believe that they were detained by federal immigration officers, campus police spokesman Larry McKinnon said Thursday.

Friends and family described their lack of communication as out of character.

“It doesn’t feel normal,” Md. Rakibuzzaman, a graduate student at the University of Georgia and a close friend of Limon, told Tampa television station WFLA. “It’s a pretty complex puzzle to me.”

Zubaer Ahmed, Limon’s younger brother, told WTVT in Tampa that they last spoke three days before he disappeared, and that Limon told him he was very busy working on his thesis, which involves using generative AI to study Florida’s shrinking wetlands. Ahmed said Limon and Bristy were discussing a potential future marriage, but were focusing on getting through their degrees first.