NEW ORLEANS — In the four years since the blowout on the Deepwater Horizon oil rig killed 11 workers and sent millions of barrels of oil gushing into the Gulf of Mexico, BP has spent more than $28 billion on damage claims and cleanup costs, pleaded guilty to criminal charges and emerged a shrunken giant.

But through it all, the company has maintained that it was not chiefly responsible for the accident, and that its contractors in the operation, Halliburton and Transocean, should shoulder as much, if not more, of the blame.

On Thursday, a federal judge here for the first time bluntly rejected those arguments, finding that BP was indeed the primary culprit and that only it had acted with “conscious disregard of known risks.” He added that BP’s “conduct was reckless.”