BP may face criminal charges over the explosion at its giant Texas refinery that killed 15 workers last March.
Proper procedures were not followed before the explosion
The US Department of Labour has referred the case to the Department of Justice, which will now decide whether to pursue it.
The news came on the same day that BP issued its final report on the blast at its Texas City refinery near Houston, which also injured 170 people.
BP said "management failures" and "employee mistakes" were responsible.
The oil company said that the working environment at the refinery "had eroded to one characterised by resistance to change, and lacking of trust, motivation and a sense of purpose".
It has set aside $700m (£400m) to compensate the victims of the explosion.
Hydrocarbon vapour cloud
The explosion happened when a hydrocarbon vapour cloud released by a unit used to produce octane for petrol was ignited, possibly by an idling pick-up truck.
BP has set aside $700m in compensation for victims of the blast
It is thought that all of the employees killed were working in temporary trailers situated too close to processing equipment at the refinery.
The blast rattled buildings up to five miles away.
The referral by the Labour department follows a US Occupational Safety and Health Administration investigation into the explosion which found more than 300 violations of health and safety standards at the refinery.
BP paid a fine of $21.3m in settlement with the OSHA last September, but has not admitted to the violations.
Neither BP or the Justice Department would comment on any possible criminal proceedings.
BP's biggest refinery
Also on Friday, BP said it planned to spend $1bn on the refinery over the next five years.
The Texas City complex has a capacity of 460,000 barrels of oil a day, pumping out up to 11 million gallons of petrol.
It has suffered previous safety problems.
In March 2004, it was evacuated after an explosion that cost the company $63,000 in fines.
In September of that year, two workers died and another was seriously injured when they were scalded by superheated water that escaped from a high-pressure pipe.