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 survived. She was in critical condition at a hospital.
"This is a devastating tragedy for the families of all the victims," State Police Superintendent Terrence Sheridan said.
Killed in the crash were pilot Stephen Bunker, 59; flight paramedic Mickey Lippy, 34; emergency medical technician Tanya Mallard, 39; and 18-year-old Ashley Youngler.
Bunker retired from the State Police after 26 years and had been a civilian pilot for the unit since 1989. Lippy had been with the State Police for four years.
Youngler and Jordan Wells, both of Charles County, were involved in the traffic accident.
It was foggy and rainy in the area about the time of the crash.
The helicopter was headed on a roughly 25-mile trip from the traffic accident north to the hospital when the crew radioed just before midnight Saturday that they wanted to land at their hangar at Andrews Air Force Base because of bad weather. Ambulances were sent to the base, which is about halfway between the accident site and Prince George's county hospital, but the helicopter never arrived.
The medevac disappeared from radar and officials lost radio contact. Two hours later, a police officer found the wreckage on a trail in a wooded area of Walker Mill Regional Park, about seven miles north of the base.
The National Transportation Safety Board and the Federal Aviation Administration were investigating.
The last fatal medevac crash for the Maryland State Police Aviation Command was in 1986 when a helicopter went down in fog in West Baltimore, killing two troopers who had just transported a shooting victim to a hospital.
The unit had two other fatal helicopter crashes in the 1970s. It began medevac missions in 1970 and has since transported over 120,000 patients, none of whom have been billed for the service, according to the unit's Web site.
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. Nine of them are more than 18 years old, including the second-oldest Trooper 2 that crashed Sunday. The helicopter, purchased in 1989, had an inspection Wednesday, Sheridan said.
State Police have defended the helicopter command, and the audit noted that the operation is highly regarded throughout the nation and has an "impeccable" safety record.
State lawmakers are likely to face major budget cuts in the coming months that could threaten a three-year plan to replace the helicopters.