A device partially exploded inside a car at the entrance to Victoria Square car park on 24 November.
A masked gang hijacked a taxi in the north of the city, placed a bomb on board and ordered the driver to take it to the shopping centre.
It exploded as the Army prepared to examine it.
Two suspects, aged 44 and 45, were arrested on Wednesday in north Belfast for questioning.
The bomb contained containing 132 lb of home-made explosives packed into a beer keg.