(CNN) -- The FBI is mourning the deaths of two special agents killed Friday during a training exercise off the coast of Virginia Beach, Virginia.
The agents, Christopher Lorek and Stephen Shaw, were assigned to the FBI's Critical Incident Response Group. The FBI said the cause of the incident is under review.
"We mourn the loss of two brave and courageous men," FBI Director Robert S. Mueller said in a statement issued Sunday.
"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."
A law enforcement official said the agents were killed while training to use a rope for landing on a ship from a helicopter.
The official said the helicopter encountered weather difficulties and the agents fell into the water rather than making a landing on the ship. The official believed winds were a factor but did not know if it was raining at the time.
The official said the agents died as a result of the impact rather than drowning.
The accident took place about 12 nautical miles off shore, the official said.
Special Agent Lorek, age 41, joined the FBI in 1996. He is survived by his wife and two daughters, ages 11 and 8. Special Agent Shaw, 40, joined the FBI in 2005 and is survived by his wife; a daughter, age 3; and a son, age 1.
The Hostage Rescue Team is part of the Tactical Support Branch of the FBI's Critical Incident Response Group. It is based at the FBI Academy in Quantico, Virginia.
CNN's Carol Cratty contributed to this report.