The reason the Elections Department (ELD) and the police took action against various persons and websites for their online posts on Cooling-Off Day for the Bukit Batok by-election was set out in a joint statement last night.

It came amid criticism from netizens about the way the police are reportedly conducting the investigations, and a call from the Workers' Party (WP) for the police to be consistent in their probes.

The statement noted that the rules for Cooling-Off Day, introduced in 2011, have been breached in the various elections.

The approach taken by the ELD has been to give warnings. In some cases, the breaches were found to be unintentional, it added.

However, "we observed what appeared to be deliberate and serious breaches of the rules... during the Bukit Batok by-election", it said.

This despite the ELD making public past breaches as well as publicising the rules several times, including issuing an advisory before Cooling-Off Day and a subsequent reminder on Cooling-Off Day itself against activities that are deemed election advertising, said the joint statement.

In the Bukit Batok by-election, various persons and sites published online posts in contravention of the rules. It said The Independent Singapore, a socio-political website, continued to post such material even after receiving a specific reminder from the Assistant Returning Officer not to do so.

"Given the blatant disregard of the (rules) in the Bukit Batok by-election, the Assistant Returning Officer decided to make police reports so that the police could investigate."

It added: "As part of the investigations, the police need to examine for evidentiary purposes electronic devices used to publish the online postings. As such, these devices had to be seized."

When the investigations are completed, the police will make their recommendations to the Attorney-General's Chambers, said the statement.

Besides The Independent Singapore, blogger Roy Ngerng and former political detainee Teo Soh Lung are being investigated as both made Cooling-Off Day posts in support of Singapore Democratic Party candidate Chee Soon Juan.

Mr Ngerng and Ms Teo said on social media they were interrogated for long hours by the police on Tuesday and their homes were also "raided". They added that their phones and computers were confiscated.

The WP urged the authorities to ensure consistency and proportionality are applied to all investigations. It also said others who were reported for similar violations in the past "were not known to have faced the same lengthy interviews, and searches and seizures of personal equipment".

Asked to elaborate, it told The Straits Times they include two People's Action Party (PAP) candidates in previous general elections. They were Dr Vivian Balakrishnan last year and Ms Tin Pei Ling in 2011. Both were reminded by the ELD of the election rule, and the posts were removed the same day.

In a Facebook post last night, PAP MP Tan Wu Meng referred to the WP statement and said WP central executive council member and Non-Constituency MP Leon Perera has been linked to The Independent Singapore, and "this was not declared in WP's statement".

"Given this, WP should be aware that it could be seen as having an interest in the outcome of the investigation, and that every effort should be made by WP to avoid the impression that WP is trying to interfere with the investigation," he said.

The Independent Singapore lists Mr Perera as a member of its advisory board. These advisers are not involved in operating the website.