At least one of the seven people involved in the alleged protest while riding an MRT train on the North-South Line on June 3 has been questioned.
Mr Sean Francis Han, 22, was in the group that was pictured wearing blindfolds while holding copies of a book titled 1987: Singapore's Marxist Conspiracy 30 Years On.
An online post described it as a silent protest of Operation Spectrum, an internal security operation that saw 22 activists arrested in 1987 in what the Government called a Marxist plot aimed at overthrowing it.
Mr Han, an activist and the editor of wakeupsg.com, was at the Cantonment Police Complex around 9.30am yesterday.
Speaking to The Straits Times (ST) after the police had finished interviewing him at about 12pm, he said: "I believe it's something to do with the police report made, so it's part of their due diligence. I tried asking but he (the investigator) said he couldn't tell me the reason."
Mr Han, who spoke outside the police complex, added that he is not sure when the rest of the group involved in the alleged protest would be questioned by the police.
ST has learnt the others include former United World College of South East Asia student Moana Ravi Jagasia, non-governmental organisation director Vanessa Ho, educator Lisa Li, Nanyang Technological University student Muhammad Zaqeer Mohd Radzi, social activist Jolovan Wham and social worker Jevon Ng.
Mr Wham had posted pictures of the alleged protest on Facebook.
The book was launched by some former ISA detainees last month.