Bob Saget's death is creating a mystery for many experts, who wonder how he suffered such a severe skull fracture ... and what exactly he hit -- but authorities are firm on what didn't happen, and they think the injury was caused by something connected to the bed.

As we've reported, Bob died from a brain bleed after hitting his head on something. Sources with direct knowledge of the investigation tell us the Medical Examiner and police are clear there was no foul play, and they're certain the blow to his head had to happen in the hotel.

We're told the injury to his head was severe enough that they're sure he could not have made the 2-hour drive from Jacksonville to the Ritz-Carlton in Orlando.

But, here's the mystery ... some medical experts say the skull fracture is consistent with being hit by a bat, or falling as far as 30 feet. Obviously, that kind of fall didn't happen, but as for Bob being struck -- we're told the electronic front door lock to his hotel room shows no one else entered the room. Once Bob entered, the next time the door opened was when hotel staff found him in the afternoon.

Our sources say the authorities say it's most likely Bob struck a portion of the bed's headboard that is not padded. We're told it's wood, and the best guess is he hit his head. Since he was under the covers, they believe he quickly lost consciousness. What's interesting ... although authorities on the scene felt the headboard scenario was the most likely, the Medical Examiner never mentioned it.

Rather, the M.E. theorizes it was an "unwitnessed fall backwards."

We're told the authorities who discovered Bob's body did not see a bruise on his head. We're told the bruise was only discovered during the autopsy.

Saget's family has filed a lawsuit in Orange County, Florida to block the release of photos from the hotel room -- because under Florida law police could go public with the images.