The FBI this week revealed that two special agents died during a training exercise on Friday, May 17.

Special Agent Christopher Lorek, 41, and Special Agent Stephen Shaw, 40, died while training off the coast of Virginia Beach, Virginia. The two were part of the FBI’s Hostage Rescue Team, an elite group within the bureau’s Critical Incident Response Group, which is based out of the FBI Academy in Quantico, Virginia. The cause of the deaths has not been released, and the FBI has stated that the incident is under review.

“We mourn the loss of two brave and courageous men,” said Robert Mueller, FBI director. “Like all who serve on the Hostage Rescue Team, they accept the highest risk each and every day, when training and on operational missions, to keep our nation safe. Our hearts are with their wives, children, and other loved ones who feel their loss most deeply. And they will always be part of the FBI Family.”

Lorek was a 17-year veteran of the FBI, having joined in 1996. Shaw had joined the FBI in 2005.

The FBI describes the Hostage Rescue Team as a national counterterrorist unit. The group responds to “extraordinary” hostage crises and other law enforcement situations within the U.S.