Natural Resources and Environment Minister Kasem Sanidwong na Ayudhaya said tests showed that the sugar boat was the main cause of pollution, but he did not rule out contributing factors.

Kasem said that salvagers did not remove the sugar but threw it directly into the river. Bacteria in the river then soaked up oxygen to "digest" the sugar and the loss of oxygen killed the fish, he said.

The test found that the dissolving of the sugar did not occur at the accident scene but at a spot further downstream, he added.

Kasem said the water at Angthong had returned to normal.

The minister said officials who visited the KTMGS factory on Wednesday evening had informed him the factory had a "closed" wastewater treatment system and did not discharge wastewater into the river.

He said officials had also been dispatched to inspect all factories in the area for analysis.

Meanwhile, deputy public health minister Dr Morakot Kornkasem said the ministry's tests found no trace of pesticides or harmful amounts of heavy metals such as lead, mercury, and cadmium in fish samples collected from Angthong.

The amount of heavy metals was very low so people could eat the fish, but it should be cooked well, he said.

Meanwhile, Angthong and Ayuthaya residents continued their protest in front of KTMGS, which they still believe to be the polluter.

At 9.30am, Angthong governor Wiboon Sanguanphong received a list of the residents' demands: urgent assistance for the affected fish farmers; a quick and transparent probe into the cause of the pollution; punishment of the wrongdoer; and prevention of recurrences.

The residents also called for it to be made clear which agencies are responsible for handling such crises.

Suchin Charoenjit, a representative of the villagers, said the inspection on Wednesday had found a leaking underground pipe but the factory owner refused further excavation. The deputy Angthong governor appeared to side with factory managers, he said.

A joint committee - comprising provincial officials, affected farmers and KTMGS representatives - was set up yesterday to thoroughly check the factory's blueprints.

Wiboon told affected farmers the province would provide initial assistance of Bt24,000 for each case.

KTMG executive Wichai Sutthilertworakul said he was sorry for what happened but insisted that it had nothing to do with the factory. He added the factory would fully cooperate with the inspection and present all documents on Monday.

Meanwhile, Bangkok Metropolitan Administration Governor Apirak Kosayodhin said canals where the polluted water was expected to flow through would be blocked before it reached the capital, which is expected on Sunday.

 He said that the amount of oxygen in the river as it flows through Bangkok was 3.6 milligrams per litre, which was considered normal.

The BMA also instructed all districts to ensure wastewater treatment systems in factories did not release wastewater into waterways.

In Pathum Thani, officials noticed there were fewer fish swimming near the Wat Hong Pathummawat river-front. They collected some for tests.