Federal investigators on Wednesday grilled the captain, surviving crew members and the owner of the scuba diving boat that caught fire and sank off of Southern California, killing 34 people.

Detailed questioning took place over several hours as the feds are still trying to piece together how the tragedy unfolded early Monday morning near Santa Cruz Island, according to Jennifer Homendy of the National Transportation Safety Board.

Homendy refused to disclose any details from the interviews.

The captain and four crew members on deck were the only ones to survive when the 75-foot vessel — called the Conception — went under during a three-day diving excursion.

Everybody else on board was asleep below deck and trapped when the blaze broke out, officials said.

Crew members were also given alcohol and drug tests, said Homendy. The alcohol tests results came back negative, while the drug test results are still pending.

The doomed boat’s fire protection systems will also be checked, officials said.

With Post wires