Two FBI agents from an elite counterterrorism unit based in Northern Virginia were killed Friday during a training exercise in the Virginia Beach area, the FBI said Sunday.

The agents were part of the FBI’s Hostage Rescue Team, which is part of the Critical Incident Response Group, and is based at Quantico.

The agents were identified as Christopher Lorek, 41, and Stephen Shaw, 40. The FBI said the incident occurred off the coast of Virginia Beach, but gave no further details. An FBI spokeswoman in the Virginia Beach area said the deaths did not involve gunfire.

The FBI said in a statement that the cause of the incident was “under review.”

The deaths brought to at least four the number of agents killed during Hostage Rescue Team training since the group was established in 1983 as a national counterterrorist unit.

TV station WAVY, based in the Tidewater/Hampton Roads area, quoted a Navy spokesman as saying that the accident happened aboard a Military Sealift Command ship that the FBI had leased for training.

At the Sealift Command headquarters in Washington, inquiries were referred Sunday night to the FBI. An FBI spokeswoman said Sunday night that the bureau had released no information beyond the statement posted on its website.

A Virginia Beach police spokeswoman said she had no information about the incident. An investigator in the state medical examiner’s office for the Tidewater District said no cause of death could be obtained before Monday.

The two agents were brought by helicopter Friday to Sentara Norfolk General Hospital, the highest-level trauma center for the area, a spokesman for Sentara Healthcare said. He could provide no information about their treatment or injuries.

Lorek joined the FBI in 1996, the bureau said, and is survived by his wife and two daughters, ages 11 and 8.

Shaw, who joined the FBI in 2005, is survived by his wife, a 3-year-old daughter and a 1-year-old son.

FBI Director Robert S. Mueller III issued a statement mourning the loss of “two brave and courageous men.” Like the others on the hostage rescue team, he said, they accepted “the highest risk each and every day,” whether on missions or in training.

“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,” Mueller said.

According to the FBI Web site, the rescue team responds to the “most complex and urgent FBI cases in the United States and abroad.

The team is based at the FBI Academy in Quantico, which is on the Marine Corps base there, about 40 miles south of Washington.

A 2006 federal report described a key capability of the hostage rescue team as the capacity to “fast-rope,” in which an assault team rappels from a helicopter. The report said the technique is particularly useful in assaulting a maritime target because it allows the FBI to place a team aboard a ship quickly.

Fast-roping was described as an advanced skill requiring “great coordination’’ between helicopter pilots and the assault teams.

The FBI Web site indicated that the unit places great emphasis on “extensive, continuous” training.

The first of the two previous deaths to occur in rescue team training came in April 1986, less than three years after the unit was established. James K. McAllister died after falling from a helicopter during training at the FBI Academy, according to the bureau.

According to the bureau’s Web site, Gregory J. Rahoi, a supervisory special agent, was accidentally shot and fatally wounded Dec. 6, 2006, at Fort A.P. Hill, in Virginia’s Caroline County. Rahoi was killed during a live-fire training exercise intended to prepare hostage rescue-team personnel for deployments to Iraq.

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