US authorities are considering charging BP managers with manslaughter after decisions they made before the Deepwater Horizon oil well explosion last year killed 11 workers and caused the biggest offshore spill in US history.

Sources close to the process told Bloomberg that investigators were also examining whether BP's executives, including former chief executive Tony Hayward, made statements that were at odds with what they knew during congressional hearings last year.

The US justice department opened criminal and civil investigations into the spill last June. The department filed a civil lawsuit against BP in December and has not filed criminal charges. An official at the department told Reuters these charges could include manslaughter, but the official declined to confirm this was under consideration.

According to Bloomberg, authorities are investigating BP managers who worked both on the rig and onshore to determine if they should be charged in connection with the workers' deaths. The investigation aims to determine whether decisions by BP managers to cut costs and increase speed on the project led to fatal safety sacrifices.

As well as the testimony of Hayward and others before congress, investigators are reviewing emails and other documents to determine what BP officials and its partners in Deepwater Horizon knew when they testified last June.

In January, a presidentially appointed national commission filed its report on the Deepwater Horizon spill and concluded that the "explosive loss" could well have been prevented. In a final report Fred Bartlit, chief counsel of commission, laid considerable blame on BP.

Bartlit said BP had been aware of problems with lab tests of Halliburton cement used to seal the well for three years. He said BP decided not to install a safety device known as a lockdown sleeve in order to save $2m (£1.2m) in costs. He also said BP's well-site leader missed a critical test known as a negative pressure test that indicated something was wrong, a test he should have supervised.

Last June US attorney general Eric Holder promised to "prosecute to the full extent any violations of the law" his investigations uncovered relating to the spill. Holder declined to name specific charges but said the justice department would be reviewing the Clean Water Act, the Oil Pollution Act of 1990, the Endangered Species Act, and "other traditional criminal statutes", a statement interpreted to refer to possible manslaughter charges.

BP has committed $20bn to settle claims by businesses and individuals who were hurt by the oil spill and has already paid out more than $4bn.

BP shares closed down 2.2% in London after the reports against a 0.3% drop in the Stoxx Europe 600 Oil and Gas index .

BP declined to comment.