Two men who were killed on a North Sea oil platform were suffocated, safety inspectors have said.
An investigation into the deaths is continuing
Sean McCue and Keith Moncrieff died after gas escaped on Shell's Brent Bravo installation.
The Health and Safety Executive inquiry has been focussing so far on temporary pipework repairs, the work permit system and operation of control valves.
Shell will not be allowed to resume production on the installation until it meets a number of conditions.
Brent Bravo is located about 180 miles east of the Shetland Isles.
Mr McCue, 22, of Kennoway in Fife, and Mr Moncrieff, 45, from Invergowrie, Tayside, were overcome by gas while working on patched pipework in a leg of the platform two weeks ago.
The HSE said the inquiry into the deaths had been given its "highest priority".
"This investigation... is being given all the inspector, specialist and laboratory resources necessary to establish the root causes of this incident as quickly as possible.
"HSE has already taken possession of certain items of plant and equipment for further tests.
"The bereaved families are being kept informed of progress," said a spokesperson.
Inspectors served an enforcement notice on Shell Expro on 15 September which stopped the company from using of specific parts of the plant.
Four days later a further notice was served which prohibited all production and processing of hydrocarbons on the Brent Bravo platform
until certain conditions are met.
"HSE acknowledges the prompt and comprehensive actions which Shell has
initiated on all their offshore and downstream installations across their European operations as a result of this incident and the high level of co-operation between HSE, the police and Shell," the spokesperson said.
"The investigation is continuing and it is HSE's intention that a report will be submitted to the procurator fiscal within three months."