Image copyright Johor Fire and Rescue Department Image caption A mile-long stretch of Malaysia's Sungai Kim Kim river became the heart of a poisoning scandal

On the morning of 6 March, an unassuming tanker lorry trundled down a dusty dirt road in southern Malaysia.

Its driver was heading for Sungai Kim Kim, a river snaking through the Pasir Gudang industrial town in Johor state. But what would happen next would expose just how vulnerable local communities are in the area.

"Based on our investigations, around 20 to 40 tonnes of chemical waste was illegally dumped into parts of the river," said Mohammad Ezzani Mat Salleh from the local environment ministry.

Mr Ezzani and his team discovered traces of toxic oil waste commonly used in "marine engine compressors".

Fire and rescue officials also identified at least 15 different types of chemicals - which included the colourless and extremely poisonous hydrogen cyanide.

Direct exposure to these hazardous fumes is extremely harmful. And the effects were harsh and swift.

Image copyright EPA Image caption Teams on the ground found traces of poisonous chemicals in air samples

A day later, 35 people - mostly students - were sent to hospital after breathing in the gases. Two schools near the site were closed.

On 8 March, emergency workers in protective hazmat suits were called in.

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The tally of victims spiralled alarmingly, and within a week more than 2,700 people were reported to have fallen ill. Victims complained of nausea, shortness of breath and vomiting spells. Hundreds of them needed immediate medical attention and had to be rushed to hospital.

Their health is still being closely monitored. As of 20 March, two patients remain in intensive care.

Image copyright EPA Image caption Students and staff from schools in the proximity were among the worst hit

Children were among the worst affected. Some 111 schools remain indefinitely closed.

Twelve days on from the incident, Malaysian Education Minister Maszlee Malik said it wasn't clear when they might be reopened.

"It is still dangerous. Teachers [are not required] to be on duty. We are monitoring the situation first," he said.

Image copyright AFP Image caption An emergency team at Pasir Gudang prepares clean-up materials

Illegal and unauthorised waste dumping takes place in many parts of Malaysia - out in the open and on the sly. The Malaysian Nature Society group said that while "good guidelines to control and regulate chemical waste" were in place, they were often difficult to enforce.

"Our volunteers are very concerned about the Pasir Gudang incident," president Ahmad Ismail told BBC News. "The pollution has not only affected public health but also impacted the rich river biodiversity of the Johore Strait. It has disturbed Sungai Kim Kim's ecosystem."

He added that the clean-up was being strictly controlled by local authorities.

Image copyright EPA Image caption Conservation groups say the ecosystem has been severely damaged by the pollution

Earlier this year a BBC investigation in the sleepy Selangor town of Jenjarom found residents still suffering from the effects of their town being smothered in 17,000 tonnes of waste - most of it, they claim, from foreign countries like the US, UK and Japan.

In Pasir Gudang, recovery efforts continue. But new signs of waste dumping have already emerged. A report from Malaysian paper The Star found construction waste still being dumped and discarded in close proximity to the river.

The dumping is done at night, it reported, with houses along the riverbanks being turned into "landing points".

Nine men face jail

In an official statement, Malaysian police said nine people had been arrested over the chemical pollution scandal.

"We cannot reveal their age and where they were detained as this could jeopardise our investigation," said Inspector General Mohamad Fuzi Harun.

If convicted, the men face jail terms of up to five years and a fine of up to RM500,000 ($122,665; £92,420).

Image copyright AFP Image caption Malaysia's prime minister said the government would examine the country's anti-pollution law

Last week Malaysia's Prime Minister Mahathir Mohamad said his government would not rule out reviewing the country's anti-pollution legislation - the Environmental Quality Act 1974 - in light of the Pasir Gudang incident.

"We will study whether the Act should be strengthened," he told reporters while visiting victims at the Sultan Ismail Hospital.

"We usually face water or solid-based contamination, but this is gas contamination - something we have not dealt with."

'How could we let this happen in broad daylight?'

Across Malaysia, people are asking the same question: How was this poisoning allowed to happen?

Social media was flooded with support for the residents of Pasir Gudang - but also with frustration and anger.

Many, like Rayson Lee from the capital Kuala Lumpur, condemned the "blatant pollution" that took place "in broad daylight" and called for stiffer penalties.

"Malaysians don't care about our country. That is why environmental perpetrators get away," the marketing executive wrote in a top-rated comment on Facebook, which drew hundreds of likes. "We don't give our environment the respect it deserves - we hardly recycle, don't think much of traffic jams and don't pay attention to the way corporations fell land to make way for highways and palm oil plantations."

Image copyright EPA Image caption Though the government says the site is now clean, it remains under close watch

On 19 March, Malaysia's Environment Minister Yeo Bee Yin announced that the polluted stretch of the Sungai Kim Kim river was now safe.

She said 900 tonnes of soil and 1,500 tonnes of polluted water had been cleaned in the 1.5km (1 mile) area.

Officials will monitor the scene for the next fortnight to help allay fears for public safety.

The state government has allocated RM6.4m ($1.5m; £1.1m), and the federal government RM8m ($1.9m; £1.4m), to clean up the consequences of that single lorry's waste.