Gardai in Letterkenny, Co Donegal, are investigating the second arson attack within the past three weeks on Master Cleaners premises on the outskirts of the town.
Three people in an upstairs flat had a narrow escape when petrol bombs were thrown into the building. They were quickly extinguished by the fire brigade after the alarm was raised by a passerby.
Forensic experts yesterday began an examination of the building and a Garda source confirmed that a number of lines of inquiry are being followed.