It was foggy and rainy in the area about the time of the crash, said National Weather Service meteorologist Jim Decarfel.
Maryland State Police fly the medical helicopters in the state, and it was their first deadly crash since 1986 when a helicopter went down in fog in West Baltimore.
Just before midnight Saturday, the helicopter reported bad weather on the way to a Prince George's county hospital and the crew said it needed to land at Andrews Air Force Base, where it was to meet ambulances to take the accident victims the rest of the way to the hospital, authorities said.
The helicopter then disappeared from radar and officials lost radio contact. Public safety officials found the wreckage about two hours later on a trail in a wooded area of Walker Mill Regional Park.
The National Transportation Safety Board and the Federal Aviation Administration were investigating.
Maryland State Police Aviation Command flies 12 twin-engine helicopters out of eight bases across the state. The last fatal crash in 1986 killed two troopers who had just transported a shooting victim. The command had two other fatal helicopter crashes in the 1970s.
A recent state legislative audit faulted the police agency for failing to document maintenance needs and costs and failing to comply with federal and manufacturer repair orders. Nine of the choppers are more than 18 years old.
But state police have defended the 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.