The crew members who survived a fire on the dive boat Conception have submitted to intensive questioning as the National Transportation Safety Board investigates the cause of the deadly blaze, which  killed all 34 passengers.

In an effort to determine what caused the blaze that engulfed the vessel early Monday morning — and why none of the passengers were able to escape — NTSB agents spent hours questioning the boat’s owner, captain, and crew, the New York Post reports.

The five crew members who were able to escape the fire also reportedly submitted to drug and alcohol testing, NTSB investigator Jennifer Homendy told the newspaper. While the results of the drug tests are still pending, the alcohol test results were negative.

Homendy told CBS News that the agency is conducting a “very lengthy, detailed, comprehensive investigation” into the deadly fire. The NTSB is expected to release a preliminary report in 10 days, but that report will not include a cause of the fire.

Investigators will examine how and why the passengers sleeping below deck were not able to escape the fire; as multiple reports have indicated that the flames blocked two escape hatches. According to the Los Angeles Times, when the NTSB examined a similar vessel, Homendy noted that it was difficult to access light switches in the small, dark space; and said she was concerned about the size and accessibility of the escape hatches.

All but one of the 34 bodies have been recovered, and investigators will use DNA testing to identify the remains.

[Feature image: Santa Barbara County Fire Department via AP]