Two FBI agents who died while training off the Virginia Beach coast fell to their deaths when a helicopter had trouble during a "maritime counterterrorism exercise," an agency spokeswoman said Monday night.

Special Agents Christopher Lorek and Stephen Shaw were killed when they fell into the water Friday. Both were members of the bureau's elite hostage rescue team, a group known most recently for rescuing an Alabama boy from a kidnapper in an underground bunker.

"The FBI agents were participating in a maritime counterterrorism exercise involving helicopters and a ship," Special Agent Ann Todd, an FBI spokeswoman, wrote in an email Monday night. "The agents were in the process of fast-roping from the aircraft onto the ship when the helicopter encountered difficulties. The agents tragically fell a significant distance and suffered fatal injuries."

It will likely be weeks before a formal cause of death is determined because the state medical examiner's regional office is waiting for toxicology results.

The team is trained in military tactics and outfitted with combat-style gear and weapons. Some of their preparation consists of scuba diving, dropping quickly out of helicopters and battling in close quarters.

"They're really the best of the best as far as civilians. Their only counterpart would be something like Navy SEAL Team 6 or U.S. Army Delta," said Clint Van Zandt, a former FBI hostage negotiator who deployed with the rescue team. "There is no other police or FBI SWAT team that's their equal, because that's their full-time job. That's all they do is train for highly critical terrorist, hostage and criminal situations."

Dale Gauding, a spokesman for Sentara Healthcare, said the men were brought to Sentara Norfolk General Hospital in a helicopter, but he declined to say what their conditions were upon arrival, deferring questions to the FBI. The hospital is the area's only Level I Trauma Center and burn trauma unit.

The Coast Guard said it was not involved, although it was notified about 11:30 p.m. on Friday. The incident occurred about 12 miles from the coastline.

The hostage rescue team is organized into tactical units made up of assaulters and snipers who are supported by helicopter and intelligence and communication teams, among others. Unlike FBI SWAT teams that train several days a month, the hostage team preps full time.

They are headquartered at the FBI Academy in Quantico in northern Virginia, although they train around the U.S. and can be deployed quickly anywhere.

In 2011, two team members helped apprehend a Somali man who prosecutors say is the highest-ranking pirate whom federal officials have ever captured. Mohammad Saaili Shibin was the chief negotiator for a group of pirates who took four Americans hostage aboard their yacht and later killed them. Unlike the other pirates in the case, Shibin was arrested in Somalia. In August, a federal judge sentenced Shibin to a dozen life sentences.

Team members also responded to the 1998 U.S. Embassy bombings in East Africa and have been deployed to Iraq and Afghanistan.

Domestically, their resume includes rescuing nine hostages held at a federal prison in Talladega, Ala. in 1991 by Cuban inmates who were rioting to prevent their return to Cuba. In February, members of the team rescued a 5-year-old boy from a small underground bunker where he was being held hostage by a 65-year-old man. The kidnapper was killed by agents.

The group was formed 30 years ago in preparation for the 1984 Olympics in Los Angeles. They have participated in hostage situations more than 800 times in the U.S. and elsewhere since 1983. Fewer than 300 people have been chosen for the unit since its creation.

The FBI has sought out former special warfare operators to join its ranks to meet the high demand for its services since Sept. 11. Still, even former Navy SEALs and Army Rangers must serve for two years as investigative special agents before they can try out to be on the team.

"These are not hit squads. These are not mercenaries. These are people who come into the FBI first and foremost to be an FBI agent. They have to have a college education, worldly experience, verbal skills," Van Zandt said. "They have to have everything we would see in a regular FBI agent plus more."

Lorek, 41, and his wife and two daughters joined Three Chopt Church of Christ in suburban Richmond about six years ago, minister Bob Odle said.

"They are as solid as they come," Odle said. "They were here every time the doors were open."

Church members knew he had a high-risk job and was often out of town, but they didn't know exactly what he did.

Someone who answered the phone at the Lorek residence Monday said the family would not be making a statement.

Lorek graduated from Texas A&M University in 1993 with a degree in ocean engineering. He joined the FBI in 1996.

Shaw, 40, joined the FBI in 2005. He is survived by his wife, a daughter and a son.

The FBI will be holding a private memorial service for the agents Tuesday.