The investigation into a fatal military helicopter crash off the Caribbean island of Aruba has been hampered after the aircraft sank into the ocean.

The Ministry of Defence said two of the four plastic balloons keeping the wreckage afloat had burst, making recovery much more difficult.

The NH-90 helicopter ditched in the sea at around 8.30pm Dutch time on Sunday. killing 34-year-old pilot Christine Martens and 33-year-old tactical coordinator Erwin Warnies. The two other crew members were not seriously hurt, a spokesman for the ministry said.

Director of operations Boudewijn Boots said the flight recorder had been recovered and was in good condition. So far the navy has not speculated on possible reasons for the crash or why two of the four crew died.

‘The only thing I can say is that the two military personnel who died were sitting in the front and the others were more towards the middle of the aircraft,’ Boots said.

The Netherlands has grounded its other 20 NH-90 helicopters. Other countries, including Australia, Belgium, Sweden and Norway have restricted or suspended their use of the aircraft because of technical or maintenance issues.