A graduate student on a Yale-sponsored trip to Puerto Rico drowned and was presumed dead Saturday after he tried to save another student who was swimming, the trip’s leader said.

Brandon Brei GRD ’06 and nine other Yale students from the graduate school and the School of Public Health went swimming in the ocean near San Juan, said Durland Fish, an epidemiology professor who led the student group. The Coast Guard called off its rescue efforts Sunday afternoon following a thorough search.

The group had spent a week training to treat dengue fever at the Center for Disease Control’s laboratory in San Juan, Fish said.

The students intended to go snorkeling, but the snorkeling was not very good, so they went swimming instead, Fish said.

“It was a dangerous place for swimming,” he said.

Two students were stranded on some rocks in the ocean and were rescued by a local Puerto Rican man who was fishing recreationally in the area, Fish said.

“[The fisherman] didn’t want to go out there himself,” Fish said. “But when [Brei] disappeared he went because they weren’t going to make it out on the rocks.”

One other student, who was a varsity swimmer as an undergraduate, was able to reach the shore on her own, Fish said.

Fish said police were on the scene within 20 minutes but were not able to get into the water. He said the Coast Guard searched the rest of the day Saturday, during the night with night vision equipment, and half of Sunday before calling off their search, which included three helicopters and equipment to track the current to know where to search.

“He probably hit some rocks,” Fish said.

Brei, who was a student in the biological and biomedical sciences program, or BBS, was studying the evolution of Lyme disease spirochetes and transmission dynamics of the agent of human granulocytic ehrlichiosis. He attended the University of Massachusetts as an undergraduate and received a degree in entomology and biology. Brei’s family lives in Orange.

Public Health Dean Michael Merson sent an e-mail Monday to notify students in the Epidemiology and Public Health program and Graduate School Dean Peter Salovey sent a message to students in the BBS program. In his e-mail, Merson said Brei presented his doctoral research to the CDC staff in Puerto Rico.

Salovey said Brei died a hero by trying to save another classmate.

“I feel for his family and his friends and his classmates and adviser who were with him when this horrible, horrific thing happened to him,” Salovey said.

Merson said Brei’s death is a terrible loss.

“Brandon was a magnificent student,” Merson said. “He was a delight for all of us.”