Authorities said the band were investigated for their "actions of support for a political cause and unfurling of a foreign flag" during the band's concert, reports said.
The Infocomm Media Development Authority also said it is investigating the possible breach of a number of licence conditions and will take "necessary action" when investigations are completed, reports added.
Singapore does not allow the public display of foreign emblems without a permit or exemption, and the Palestinian flag is especially sensitive given the war in the Gaza Strip and Singapore's Muslim population.
In 2023, the home affairs ministry said: "The ongoing Israel-Hamas conflict is an emotive issue. We would like to advise against the public display and wearing of articles in relation to the conflict, given the heightened sensitivities."
"The peace and harmony between different races and religions in Singapore should not be taken for granted, and we must not let events happening externally affect this peace and harmony we have in Singapore."
Singapore's resident population is 74 per cent Chinese, 13.6 per cent Malay, and nine per cent Indian, with 3.3 per cent classified as others.
About 15 per cent are Muslim.
Massive Attack, formed in Bristol in the west of England in 1988, are known for their outspoken political stances, with videos during their concert highlighting conflicts from the US-Israeli war on Iran, to Sudan, the Gaza Strip and Ukraine.
Neither the concert organiser Lushington nor the band immediately responded to a request for comment.
