The quick-thinking of a mystery man in Essa Township is being credited with saving lives after a fire broke out at a split-level apartment.

The fire happened on Centre Street in Angus late Thursday night.

It caught the attention of a man walking by who saw the flames shooting into the night sky and ran to alert the neighbouring home, likely saving at least four lives.

The two-unit building was vacant at the time of the blaze, but the flames were shooting into the house next door where four people were sleeping.

"There's no two ways about it. That time of night, with the fire involved that we had... if they hadn't been alerted, there was a good possibility there was something more dramatic that would have happened," said Essa Township Deputy Fire Chief Doug Burgin.

Officials said both properties sustained damage.

The Fire Marshal and investigators are combing through the debris for clues as to what started the fire.