Washington (CNN) -- Two medical helicopter crashes in eights hours on Wednesday are again drawing attention to the dangers of aircraft that are intended to save lives.
In the more serious crash, all three people on a Med-Trans helicopter died when the aircraft crashed in a field near Ventura, Iowa, Wednesday evening. The chopper had been en route to pick up a patient at an Emmetsburg, Iowa, hospital, authorities said.
Earlier in the day, an Air Methods helicopter crash landed near Seminole, Oklahoma, injuring the four crew members on board. There was no patient on the aircraft.
Federal safety officials began a concerted effort to address medical aircraft safety in 2006 after an increase in the number of accidents around the country. Fifty-five medical aircraft crashed between January 2002 and January 2005, resulting in 54 fatalities and 18 serious injuries.
But problems have continued. Sixteen people died in medical aircraft crashes in 2010, another three in 2011, and three last year, according to records released by the National Transportation Safety Board.
Officials say the pressure to quickly respond in various conditions -- at night, in foul weather -- makes these operations inherently dangerous.
Wednesday's fatal in Iowa crash occurred as a helicopter for Mercy Medical Center, operated by Med-Trans, went down about 9 p.m. The dead were identified as nurse Shelly Lair-Langenbau, paramedic Russell Piehl, and pilot Gene Grell with Med-Trans.
"They were each selfless health care professionals devoted to the delivery of emergency medical care," Dan Varnum, president of Mercy North Iowa, said in a statement.
Authorities said that the helicopter did not issue a Mayday warning and that it is too early to know the cause of the crash. It is not known whether weather was a factor, but Cerro Gordo County Sheriff Kevin Pals told CNN that the blacktop road was ice-covered when he arrived at the scene about 20 minutes after the crash.
Both the Federal Aviation Administration and the NTSB are investigating.
According to an NTSB database, Med-Trans helicopters were involved in two nonfatal crashes in 2012. In both cases, the helicopters sustained substantial damage. The Dallas-based company operates about 60 aircraft at bases in 18 states, according to its website.
The Oklahoma crash involved a helicopter built in 2009 and operated by the company Air Methods.
"The pilot stated he experienced an engine failure and looked for an open area to land," Lt. George Brown, a spokesman with the state's Highway Patrol, told CNN.
The tail rotor and other parts of the tail broke away from the rest of the aircraft in the crash.
Air Methods Vice President of Corporate Development Craig Yale said he couldn't comment on the circumstances of the crash because of the active investigation but said all of the injured were stable and some had been released from the hospital.
NTSB records show the company has been involved in 12 other helicopter accidents, which included 20 deaths, since 2007.
Yale attributed the total to the high number of flights they operate and noted that four of the deaths were on another company's helicopter involved in a midair collision.
Air Methods has a dramatically lower rate of fatalities than the rest of the industry, he said, but it operates about half of the medical flights in the United States.
The company has more than 400 aircraft and operates in 48 states, according to its website.
"Air Methods has invested $100 million in safety enhancements, (including) night vision goggles, terrain avoidance systems and weather systems in the aircraft," Yale added. The company also participates in an FAA Safety Management System.