The pilot of a medical helicopter twice radioed for help in foggy weather before crashing Sunday, killing four of the five people on board in the latest of a growing number of air ambulance accidents, authorities said.

Crashes of medical aircraft have been increasing since the 1990s, in part because it is a booming business, fueled by the closing of emergency rooms in rural areas and an aging population, according to the National EMS Pilots Association. However, the state-run program in Maryland does not charge for its services, and was known for its safety record. It has had just three other fatal helicopter crashes in four decades.

"We are the only operation in the country that has the multiple mission of medevac, search and rescue, law enforcement, homeland security," State Police spokesman Greg Shipley said. "It's a very unique situation."

On Sunday, a veteran pilot, a flight paramedic, a county emergency medical technician and one of the traffic accident victims died in the crash, authorities said.

An 18-year-old woman also injured in the traffic accident in Charles County survived the helicopter crash. She was in critical condition at a hospital.

The helicopter was headed on a roughly 25-mile trip from the traffic accident to the hospital when the aircraft was diverted to Andrews Air Force Base late Saturday because of bad weather. As they approached, their runway location was changed and the pilot radioed that he was having trouble assessing his surroundings. He again asked for assistance with the landing, and that was the last air traffic controllers heard from him.

The chopper crashed about 1:15 a.m., three miles from the base.

The NTSB and the Federal Aviation Administration were investigating the cause of the crash.

The recent increase in medical helicopter accidents has triggered the safety board to hold a public hearing on the matter, Hersman said, though no date had been set.

A federal investigation in 2006 found there were 55 accidents air ambulance accidents from 2002 to 2005, prompting the safety board to issue four recommendations, including higher standards for medical aircraft and more stringent decision-making in determining whether to fly in bad weather.

Officials said the helicopter was cleared to fly by Andrews Air Force base.

Crashes in Texas, Wisconsin and Arizona, where two medical helicopters were in a fiery collision in June, have underscored the dangers of the medical flights. Some have questioned whether it would be safer to transport patients by ground ambulance.

Dr. Bryan Bledsoe, an emergency medicine physician who teaches at the University of Nevada and has researched accident rates of medical helicopters, said Sunday the Maryland medevac system has a good safety record, but medical flights are overused nationwide.

"We've just gotten into a situation here in the United States where we think that the helicopters are a panacea," Bledsoe said. "And they are an important tool, but they are just a tool. We vastly overuse them, patients don't benefit and they are expensive."

There is a tendency to fly in questionable weather, he said. In many cases, the flights aren't justified because the distance to the nearest hospital is not that great or the injuries are not severe enough, he said.

Killed in the crash Sunday were pilot Stephen Bunker, 59; flight paramedic Mickey Lippy, 34; emergency medical technician Tanya Mallard, 39; and 18-year-old Ashley Younger.

Younger and Jordan Wells, who survived the crash, were involved in the traffic accident in Charles County.

An aunt of Tanya Mallard, the medical technician killed in the crash, said she was proud of her niece's work.

"I lost someone I truly, truly love, I'm sorry for everybody else's loss," Cheri Douglas said. "My family is truly, truly hurt.'

A recent state legislative audit faulted the police agency for failing to document maintenance needs and costs for its fleet of 12 twin-engine helicopters. The helicopter that crashed was purchased in 1989, the second-oldest in the fleet, which is paid for in part by an annual charge on residents' vehicle registration. It had been inspected Wednesday, State Police Superintendent Terrence Sheridan said.

State Police have grounded all of their flights until the cause of the crash can be determined. Other states and private companies will cover Maryland in the meantime, Sheridan said.