Gardai are investigating an incident in the early hours of this morning during which nine cars were damaged by fire at Rosemount in Letterkenny's town centre.
It is understood that the alarm was raised by gardai who were on patrol.
The emergency crews were called to the scene - an underground carpark - at around 2 am and it took firefighters over an hour to bring the blazes under control.
Residents living in apartments nearby were evacuated were taken to Dillon's Hotel on Upper Main Street.
A number of businesses, including O'Hehirs Bakery & Cafe and Watson's Menswear, are closed because of extensive smoke damage.
Rosemount Lane, which links Lower Main Street with Pearse Road, remains closed and a technical examination of the scene is currently underway by a garda forensics team.
Gardai have appealed for those with information to contact them on 07491-67100.