State police chief Mohd Khalil Kader Mohd said they had requested help from their Singapore counterparts to detain the man, aged in his 30s.

Speaking at a press conference, he said eight of the 11 arrested for investigations had been released on police bail after being found to be not involved in the river pollution, Bernama reported.

Khalil said police needed proof before charging anyone in court and could not simply arrest anyone.

As such, he cautioned social media users not to hurt the feelings of policemen and other officers who were working hard to trace and nab those responsible for the incident which saw more than 5,000 people seeking medical attention after having difficulty breathing toxic fumes.

Meanwhile, a lorry driver, N Maridass, 35, pleaded not guilty in the Sessions Court to charges of dumping toxic chemicals into the river between midnight and 1am on March 7.

Two company directors, Singaporean Wang Jin Chao, 34, and Malaysian Yap Yoke Liang, 36, were accused of conspiring with Maridass. They pleaded not guilty as well.

They belonged to a used tyre-processing company.

All three were charged under Section 34B(1)(a) of the Environment Quality Act 1974 while Yap and Wang were also charged under Section 42 of the same Act.

They face sentencing under Section 34B(4) of the Act.

Maridass was allowed bail of RM100,000 on one surety while Yap was released on RM250,000 bail on one surety.

No bail was offered for Wang.

The court will mention the case again on April 30.

Wang, Yap and another director are expected to face 15 other charges tomorrow morning.