From an engineering perspective.

Cause and time of death

The crew cabin after breakup, indicated by the arrow
The crew cabin was made of particularly robust reinforced aluminum and detached in one piece from the rest of the orbiter.[20] At the time of separation, the maximum acceleration is estimated to have been between 12 and 20 g. During vehicle breakup, the cabin detached in one piece and slowly tumbled into a ballistic arc. Within two seconds after breakup the cabin had dropped below 4 g, and was in free fall within 10 seconds. The forces involved at this stage were probably insufficient to cause major injury to the crew.[17]

At least some of the crew were alive and at least briefly conscious after the breakup, as three of the four recovered Personal Egress Air Packs (PEAPs) on the flight deck were found to have been activated.[17] PEAPs were activated for Smith[21] and two unidentified crewmembers, but not for Scobee.[17] The PEAPs were not intended for in-flight use, and the astronauts never trained with them for an in-flight emergency. The location of Smith's activation switch, on the back side of his seat, indicated that either Resnik or Onizuka likely activated it for him. Investigators found their remaining unused air supply consistent with the expected consumption during the post-breakup trajectory.[21]:245–247

While analyzing the wreckage, investigators discovered that several electrical system switches on Smith's right-hand panel had been moved from their usual launch positions. The switches had lever locks on top of them that were required to be pulled out before the switch could be moved. Later tests established that neither the force of the explosion nor the impact with the ocean could have moved them, indicating that Smith made the switch changes, presumably in a futile attempt to restore electrical power to the cockpit after the crew cabin detached from the rest of the orbiter.[21]:245

On July 28, 1986, NASA's Associate Administrator for Space Flight, former astronaut Richard H. Truly, released a report on the deaths of the crew from physician and Skylab 2 astronaut Joseph P. Kerwin. Kerwin's report concluded that it is unknown whether the crew remained conscious until ocean impact, because it is unknown whether the crew cabin remained pressurized. Depressurization would have caused the crew to quickly lose consciousness, as the PEAPs supplied only unpressurized air. Pressurization could have enabled consciousness for the entire fall until impact. The crew cabin hit the ocean surface at 207 mph (333 km/h) approximately two minutes and 45 seconds after breakup. The estimated deceleration was 200 g, far exceeding structural limits of the crew compartment or crew survivability levels. The middeck floor had not suffered buckling or tearing, as would result from a rapid decompression, but stowed equipment showed damage consistent with decompression, and debris was embedded between the two forward windows that may have caused a loss of pressure. Impact damage to the crew cabin was severe enough that it could not be determined if the crew cabin had been previously damaged enough to lose pressurization.[17]

https://www.govinfo.gov/content/pkg/GPO-CRPT-99hrpt1016/pdf/GPO-CRPT-99hrpt1016.pdf