In its first investigative report regarding what caused the April 20 Deepwater Horizon oil rig explosion and subsequent massive oil spill, BP (NYSE:BP) said a combination of human, operational and engineering errors led the rig’s oil well to destabilize and the emergency blow-out preventer to fail to seal the well.

However the company deflected much of the criticism to its oil-well partners: Halliburton (NYSE:HAL) and Transocean (NYSE:RIG).

Because the report was handled by BP’s own internal investigators, the objectivity of the report will be called into question by BP's critics. The oil company is under multiple investigations to determine if it was criminally negligent, and the report's conclusions could be used to show there are too many extraneous factors happening to charge BP with a crime.

The 200-page report said eight safety, engineering or operations issues happening concurrently or subsequently eventually led to the oil rig explosion, the deaths of 11 oil workers and the millions of barrels of spilt crude oil.

According to BP, the well’s cement seal was likely the fountainhead of the problems with the well. Tests done of the well's cement mixture - engineered and provided by Halliburton - showed the mixture was insufficient for the type of well constructed and led to oil and natural gas leaking into the well at unacceptable levels.

While the seal was likely the first problem with the well, BP said, there were several consecutive happenings that exacerbated the issue. The report said the Deepwater Horizon crew – which consisted of mostly Transocean employees but did include some BP workers – failed to recognize problems with the well’s integrity 40 minutes before the blow out. 

There was also evidence that the BP crew incorrectly interpreted the results of what’s known as a “negative pressure test” that also led to the well’s growing problems going unrecognized.

If the signs that oil was seeping up the well had been noticed before that 40 minute gap, there could have been enough time to close the well appropriately, BP investigators said.

As oil continued to seep into the well, a combination oil-and-gas mixture eventually shot out of the derrick, flooding the entire platform with flammable hydrocarbons. According to BP, typical operational procedures should have dumped the hydrocarbons into the ocean via the rig’s overboard piping system.

Instead the BP-and-Transocean crew diverted the overflowing well into what’s known as a “mud-gas separator” which is not typically used in these situations and was quickly overwhelmed. That gas eventually made its way into the rig's engine room and ignited.

There has been much attention paid to the question of why the well’s emergency blow-out preventer, a complicated piece of equipment owned and maintained by Transocean designed for situations like this, never worked properly. In BP’s report, the blow-out preventer was hobbled by several problems, including incorrectly charged batteries and a defective solenoid.

Further, when the rig exploded it is believed  the wires used to control the blow-out preventer were severed making it impossible for the crew above the surface to manually control the device.

The report, which took four months to complete and involved 50 internal and external investigators at BP, will not likely be the final word on what happened on April 20. The Department of Justice is also doing its own investigation into the series of events leading up to the blow out along with ongoing Congressional investigation.

Both Transocean and Halliburton criticized the report, saying BP ignored issues like oil-well design that could be directly attributed to BP-only decisions regarding the well.

“This is a self-serving report that attempts to conceal the critical factor that set the stage for the Macondo incident: BP's fatally flawed well design,” Transocean said in a statement. “In both its design and construction, BP made a series of cost-saving decisions that increased risk - in some cases, severely.”

Transocean is currently doing its own internal investigation into the well explosion.

Halliburton also responded to the report, saying that ultimately the owner of the well - in this case BP - is responsible. 

"Halliburton remains confident that all the work it performed with respect to the Macondo well was completed in accordance with BP's specifications for its well construction plan and instructions, and that it is fully indemnified under its contract for any of the allegations contained in the report," the contractor said in a statement. "Deepwater operations are inherently complex and a number of contractors are involved which routinely make recommendations to a single point of contact, the well owner."