The owner of the drilling rig that exploded in the Gulf of Mexico in 2010 has started to present its defense at a trial designed to determine the causes of the deadly disaster and assign fault to the companies involved.

Transocean Ltd. called its first witness Monday on the 13th day of the trial.

A federal judge already has heard testimony by more than a dozen witnesses called by the Justice Department and attorneys for Gulf Coast businesses and residents who claim the spill cost them money.

The plaintiffs' lawyers still expect to call another witness to the stand this week, an employee of cement contractor Halliburton.

Halliburton and Macondo well owner BP PLC also will call their own witnesses after Transocean finishes presenting its case.