Fire investigators still don't know what caused the fire at the Byblos restaurant. (CBC)

Fire investigators still don't know what caused a devastating fire Monday night at Byblos Restaurant in Charlottetown.

The blaze destroyed the interior of the Lebanese restaurant and sent the owner to hospital in Halifax.

On Friday the restaurant parking lot was still cordoned off with police tape. Investigators have been in and out of the building, gathering evidence that may help them determine what caused the fire.

Charlottetown Police investigators want to speak to the restaurant owner, Charbel Jreij. He had to be rescued from the bathroom of the restaurant Monday night and was sent to hospital in Halifax. According to his brother-in-law, Jreij inhaled a lot of smoke and lost consciousness. He has shown some signs of improvement but remains unconscious with breathing problems.

The fire has taken a toll on three nearest businesses to the restaurant.  They all suffered smoke damage and remain closed.

Peake and McInnis Insurance has found a temporary location in West Royalty and expects to be there for the next three months.

"There's considerable smoke in the ceiling," said David McInnis. "They have to basically remove the roof from the inside and clean it and replace it."

Fire officials said when they started the investigation they knew it wouldn't be an easy one because of the severity of the damage. There's no word yet on when their investigation will wrap up.