Arsonists put lives in danger when they put material through a shop letter box.
A passer-by noticed the Rent a Video store in Arundel Road, Littlehampton, was on fire and alerted the fire brigade.
CCTV footage has revealed the blaze had been burning for 15 minutes before it was spotted. People living in a flat above the shop were unaware of the peril.
Investigations at the scene revealed the fire had been started by arsonists.
DC Mick Burkinshaw said: "They put something through the letter box but we have no idea of any motive whatsoever.
"It's very lucky the man spotted it when he did. Had it continued the people upstairs could have suffered serious injuries."
Damage inside the store was limited.
Police are appealing for witnesses to the incident, which happened at 1.50am on Thursday.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules hereComments are closed on this article