Human remains that were found in the waters of Tampa Bay this week have been identified as those of one of two University of South Florida graduate students who were reported missing last month, officials said Friday.

The Hillsborough County sheriff, Chad Chronister, said at a news conference on Friday that DNA testing had confirmed that a body that was found on Sunday in a black trash bag in a mangrove was that of Nahida Bristy.

Ms. Bristy and Zamil Limon, both of whom were 27 and doctoral students at the university, were last seen on April 16.

Mr. Limon’s body was found last week in a trash bag on the Howard Frankland Bridge in Tampa, about 20 miles southwest of his home near the university’s campus, the authorities said.

“What began as a missing persons case ended with the discovery of a monstrous crime,” Sheriff Chronister said on Friday. “Our community has been left heartbroken.”

Mr. Limon’s roommate, Hisham Abugharbieh, 26, a former student at the university, was arrested on April 24 and charged with two counts of first-degree murder.

Mr. Abugharbieh was also charged with unlawfully holding or moving a dead body, failure to report a death, tampering with evidence, false imprisonment and battery.

Ms. Bristy lived on the university’s campus but not with Mr. Limon and Mr. Abugharbieh. Both Mr. Limon and Ms. Bristy were from Bangladesh.

A motive is still unclear, Sheriff Chronister said.

When detectives began investigating the missing persons’ report, one of their first stops was a dumpster outside the building where Mr. Limon and Mr. Abugharbieh shared an apartment.

They found Mr. Limon’s glasses, student ID, wallet and clothes, “which were extremely bloodied at the time,” Sheriff Chronister said.

Investigators also searched Mr. Abugharbieh’s car and found evidence of blood, which was later identified as Ms. Bristy’s.

After executing a search warrant at the apartment that Mr. Abugharbieh and Mr. Limon shared, investigators found evidence of a large pool of blood in the kitchen area that extended into the suspect’s bedroom, Sheriff Chronister said.

Mr. Abugharbieh’s phone had been recently wiped clean, but the sheriff said that a forensic analysis revealed a search history in the days before the victims were reported missing that included prompts like “can a knife penetrate a skull?” and “can a neighbor hear a gunshot?”

Mr. Abugharbieh also had recently purchased large, heavy-duty contractor bags, duct tape and cleaning spray. “This was calculated,” Sheriff Chronister said.

Investigators believe Mr. Limon and Ms. Bristy were both killed inside the apartment. Mr. Abugharbieh had deep lacerations on his hand and shoulder that investigators believe were defensive wounds, the sheriff said.

As investigators were searching for the bodies of the missing students, members of the Mr. Abugharbieh’s family told officials that on Sunday night that Mr. Abugharbieh had “battered” and “committed domestic violence against them,” including trying to grope his sister.

Investigators discovered Mr. Limon’s remains in a large black trash bag on the side of the road on the Howard Frankland Bridge, which carries Interstate 275 across Old Tampa Bay between St. Petersburg and Tampa.

He had been stabbed several times and bound by his hands and ankles, Sheriff Chronister said.

A kayaker who was fishing in a mangrove on Monday snagged his line on something and discovered a garbage bag that looked as if it contained a human body, Sheriff Chronister said.