Updated

A medical transport helicopter that crashed in coastal South Carolina, killing all three crew members on board, was damaged last year after scheduled maintenance on the aircraft was delayed, authorities said Saturday.

The helicopter had just dropped off a patient at a hospital in Charleston and was flying to Conway, about 90 miles to the northeast, when it crashed about 11:30 p.m. Friday in Georgetown County, NTSB spokesman Peter Knudson said.

National Transportation Safety Board investigators were on the scene Saturday and didn't speculate on what caused the crash during a later afternoon news conference.

Federal Aviation Administration spokeswoman Kathleen Bergen said the pilot last radioed air traffic control at 11:05 p.m. Friday, saying the crew was about four miles from an airport near Charleston and had it in sight.

The helicopter's owner, Addison, Texas-based Omniflight, identified the crew members killed: Patrick Walters, 45, of Murrells Inlet; flight nurse Diana Conner, 42, of Florence; and paramedic Randolph Claxton Dove, 39, of Bladenboro, N.C.

A thunderstorm moved through the area shortly before the crash, according to the National Weather Service. Authorities have not said if weather conditions contributed to the accident.

On its Web site, Omniflight says the company operates 100 aircraft from 72 bases in 18 states with 1,000 employees.

"Omniflight is deeply saddened by the tragic loss of its crew members and wishes to express its deepest regrets and sincerest condolences to the families and friends of those who lost their lives," the company said in a statement.

Omniflight spokesman Joel Hochhalter says the company was suspending service in Conway because the helicopter was the company's only one in that area.

Hochhalter also said the American Eurocopter AS350B2 had never had mechanical problems, but NTSB records show the aircraft was damaged during a May 2008 incident.

According to the NTSB, a pilot preparing for takeoff from Caffery Heliport in Hiram, Ga., said the helicopter began to shake violently as he advanced the throttle. Immediately shutting down the helicopter, he told investigators he saw extensive damage to the main rotor.

An audit of maintenance records showed that the rotor should have been replaced almost 600 flight hours earlier, the report showed. Omniflight immediately audited its other helicopters, and no other rotors were shown as overdue for replacement.

NTSB records also show that another similar helicopter, also operated by Omniflight, was involved in another incident in South Carolina. On July 2, another Eurocopter was damaged when it struck steel poles adjacent to the helipad at Loris Community Hospital Heliport.

No one was injured. The pilot continued the landing and performed an emergency engine shutdown, records show.

Earlier this month, NTSB officials urged the government to impose stricter controls on emergency helicopter operators citing last year's record number of fatalities. There were nine accidents between December 2007 and October 2008, killing 35 people.

There have been three accidents since then, but no fatalities reported until the South Carolina crash.