CHICAGO, Illinois (CNN) -- The rotor blades on a critical-care helicopter that crashed -- killing all four passengers -- likely clipped a guy wire, causing the blades to break off, an air safety investigator said Thursday.
"A rotor blade is not designed to travel through anything except air," said National Transportation Safety Board investigator John Brannen.
The Wednesday night crash in Aurora, Illinois, a Chicago suburb, is the latest in a string of medical helicopter accidents that has raised concerns among NTSB officials.
Pieces of what appear to be the main rotor blades, their hubs and shaft were found in a nearby apartment complex apartment lot, about 100 yards from the main wreckage, Brannen said.
"It indicates that possibly the main rotor blades separated from the aircraft during flight," Brannen said, citing "preliminary information."
Police evacuated 16 apartment buildings near the crash as a precaution while crews assessed the tower's stability. The guy wires add stability to the tower, which Brannen said is 1,149 feet high.
Two crew members, a nurse and a 1-year-old patient were killed when the Air Angels Inc. chopper, a Bell 222, crashed in a field, authorities said.
The aircraft was en route from Valley West Community Hospital in the town of Sandwich to Children's Memorial Hospital in Chicago, said Federal Aviation Administration spokeswoman Elizabeth Isham Cory.
Investigators don't know whether they will be able to retrieve any usable data from the chopper because it caught fire, Brannen said. Weather conditions were clear when the accident occurred, he added.
"We don't have any indication at this point that there was a distress call from the helicopter," he said. Brannen said he reached the crash scene about 90 minutes after the copter went down.
Air Angels Inc. is an independent emergency medical transport service based at Clow Airport in Bolingbrook, officials said.
The FAA and NTSB are investigating the accident.
Last month, NTSB Vice Chairman Robert Sumwalt said his agency was concerned about the increasing number of medical helicopters that had been crashing.
"We have had too many of them," he said. "We need to do something about it. We need to do something about it right now."
His remarks came three days after a Maryland State Police chopper carrying two victims of an automobile accident crashed in foggy weather as it attempted to land in suburban Washington.
The September 27 crash marked the eighth fatal medical helicopter accident in a year. While the NTSB acknowledges that emergency medical operations are often conducted in darkness and bad weather and over rough terrain, the agency says the accident rate for the missions is too high.
The NTSB has raised concerns about medical helicopter crashes in the past.
The agency investigated 55 crashes -- resulting in 54 fatalities -- between January 2002 and January 2005. In a 2006 report, the NTSB said 29 of the accidents could have been prevented.
At that time, the NTSB noted some recurring themes: less stringent requirements for flights with no patients on board; a lack of flight risk-evaluation programs; lack of consistent, comprehensive dispatch procedures; and few requirements to use certain safety-enhancement technologies, such as night-vision goggles.
More than 50 people have died in medical flight accidents since the NTSB made its recommendations in 2006. Nearly two-thirds of the fatalities involved nighttime or poor-visibility conditions.
Fewer than a third of about 800 emergency medical services helicopters in the United States have night-vision technology. A lot more would like to have it, according to a survey of 382 active helicopter EMS pilots by the National EMS Pilots Association.
The survey, published in May, found that 82 percent of pilots prefer to use night-vision equipment.
"We are an independent federal agency, charged by Congress to investigate transportation accidents, to determine the probable cause and then to issue safety recommendations," Sumwalt said last month. "When those recommendations are not implemented, lives are lost, needlessly."
Although the NTSB offers recommendations, the FAA has the power to make regulations mandatory.
"We understand the NTSB safety recommendations, and we agree with all of them," said Jim Ballough, director of the FAA's flight standards service. "We also understand that rulemaking takes a long time."
CNN's Marsha Walton contributed to this report.