Norwegian officials probing the crash last month of an Airbus Group SE EC225 helicopter said Friday a “sudden catastrophic failure” downed the chopper and the search for wreckage was still ongoing.

Norway’s Accident Investigation Board, or AIBN, in a preliminary report said the helicopter appeared to have been operating normally until the problems developed within 1 to 2 seconds, at which time the combined cockpit voice and flight data recorder stopped operating. “There are no indications that flight crew actions were a factor in the accident,” the report said.

The helicopter, operated by CHC Group Ltd., crashed April 29 while flying to Bergen Airport from Statoil ASA’s Gullfaks B oil platform, killing all 13 people on board.

Norwegian and British air-safety regulators imposed a ban on all EC225 passenger flights, though Airbus said it saw no reason to ground the model. Bristow Group Inc., another helicopter operator, also idled some EC225s in Australia. CHC, which already was financially struggling before the accident, has since filed for chapter 11 bankruptcy protection.

Airbus and the European Aviation Safety Agency have since called for emergency inspections of EC225 helicopters.

Norwegian investigators said the recovered parts of the wreckage are being analyzed. The focus is on the main rotor head, which detached from the helicopter, the main gearbox and associated components.

“At this preliminary stage of the investigation, detailed metallurgical examinations have not been performed. The examinations so far have not shown any sign of fatigue failure,” the seven-page report said.

Norwegian investigators said that “a significant sea and land search is ongoing” to receive other pieces of the helicopter.

The EC225 has been involved in previous incidents linked to problems with the main gearbox. However, Airbus said early evidence suggest last month’s accident had a different cause, based on its investigators’ findings.

The EC225 is a mainstay of helicopter operations to North Sea offshore oil and gas facilities. Helicopter operators have been forced to bring in other models to provide capacity to ferry oil workers and equipment while the EC225 helicopters are sidelined