Detectives from the PSNI’s Serious Crime Branch are heading up the probe after the small was located on Tuesday night just before 10 pm.
Army Technical Officers were tasked to the building on Short Strand Road after the alarm was raised.
The ATO later declared it was a viable device and made it safe following several hours of examination.
A number of residents had to leave their homes. Albertbridge Road, Ravenhill Road and Woodstock Link were also closed.
No details about the device have been released whether it was a pipe bomb or an anti-personnel type.
However detectives have appealed for anyone who noticed any suspicious activity in the area to contact them.