Two doctoral students at the University of South Florida have gone missing, according to authorities. Family and friends of the pair, both from Bangladesh, say they haven't heard from them since last Thursday. 

Zamil Limon and Nahida Bristy, both 27, were last seen in the Tampa area on April 16, the University of South Florida Police Department said in a news release. In a statement to CBS News, the Hillsborough County Sheriff's Office, which is also investigating, said detectives are actively following leads and conducting searches in multiple locations, including Hillsborough and Pinellas counties. 

Limon is studying the use of AI in environmental science and was set to present his doctoral thesis this week, his family said. He was last seen around 9 a.m. last Thursday at his residence in Tampa, the USFPD said. Bristy is studying chemical engineering. She was also last seen on April 16, at approximately 10:00 a.m. on the USF Tampa campus, authorities said.

Loved ones say their disappearances are out of character and are concerned for their well-being. 

"It's extremely suspicious for us," said Zubaer Ahmed, Limon's brother. "How can two people... disappear out of nowhere?"

According to Ahmed, Limon had asked family members not to contact him because of his upcoming thesis presentation.  

"He was extremely busy, and Tuesday [April 14]... he also informed us that, please don't call me unless it is... [an] emergency," Ahmed told CBS News. There has been no further contact with Limon confirmed since Thursday, April 16, authorities said.

A friend of the pair told CBS News that Limon had put on a shared Outlook calendar at the lab that he was planning to work from home on the day he and Bristy went missing.

"He took a work-from-home to work on [his] thesis," said Omer Hossain, the friend. "He put it in the Outlook calendar." 

Hossain said the pair were part of the same friend group, and that friends were unaware that the two may have had a romantic relationship.

Ahmed said the pair had developed feelings, "gradually they thought about having a future together, but they preferred their study first." 

Bristy's older brother, Zahid Pranto, told CBS News that Bristy would typically be in touch with her family on a daily basis.

"There has been no single day without contact with her," Pranto said. "My family is just devastated."

According to the families, the disappearances also don't make sense because the pair already had flights booked to travel to Bangladesh for summer break.

Anyone with information is asked to contact university authorities at 813-974-2628.