As thousands of inspectors fanned out across China to check for tainted milk powder today , police arrested six more people for involvement in a scandal that has killed four and sickened more than 6,200 infants.

Six more people have been arrested in Hebei province, where the Sanlu Group that is at the heart of the milk powder contamination is based, bringing the total in custody to 18 by this morning (Thursday).

Police said six had allegedly sold the industrial chemical melamine, used in plastics and fertilisers, but mixed with milk at collecting stations to bamboozle tests for protein. The other 12 were milk suppliers accused of selling the contaminated milk.

So serious is the latest scandal to tarnish the “Made in China” brand that Premier Wen Jiabao presided at a cabinet meeting yesterday to coordinate the government’s response. The meeting concluded: “The Sanlu infant milk powder incident reflects chaos in the dairy products market and loopholes in supervision and administration that has not been vigorous.”

Three babies have died and the government of the westernmost Xinjiang region reported a fourth fatality today. Officials say 6,244 infants have been sickened after being fed the powder laced with melamine, and of those 158 were fighting acute kidney failure. More cases were expected as more and more parents take the toddlers to hospitals and as contaminated powder is found in other brands.

Already the chairwoman of the Sanlu Group has been fired and arrested and the mayor of the Hebei provincial capital has also been sacked.

But latest reports revealed the sale of the contaminated milk powder had been going on for many months – even though Sanlu had learnt of the problems.

The Hebei deputy governor said Sanlu knew that melamine was being added to its milk since as early as 2005, a Hong Kong newspaper controlled by Beijing reported. The deputy governor said 41 of 372 milk stations supplying Sanlu had been found to have problems.

One suspect told police that from February 2007 to July 2008 he bought 200 44-pound (20 kilogram) sacks of melamine for 200 yuan (16 pounds) each and sold them on to milk suppliers.

The victims of this collusion of greed and secrecy are among society’s most vulnerable. Many are peasant families who can afford only to buy formula produced by less prestigious state-owned companies or migrant workers who have left their babies with grandparents or relatives for better-paying jobs in coastal areas.

In a country with few outlets for public anger, many have vented their fury in online chatrooms. One anonymous posting read: “Do other milk products have problems? Now it’s a question of trust. If the relevant producers are not bankrupted it won’t give the food industry a shock. If the criminals aren’t killed, the masses will not be satisfied.” Another wrote: “Society today is all about money, for many you can adopt any methods.”

Better-off Chinese are crossing the border into Hong Kong to buy up imported brands of formula.

In Beijing, one young father-to-be said he was encouraging his wife to breast-feed. “I think more people in the cities understand that mother’s milk is better. But in the countryside they think there is more nutrition in milk powder.”

But many have not lost their sense of humour. One joke doing the rounds online describes a man going into a shop to buy bread only to find there is no expiry date on the package. The sales clerk gets out a pen and says: “Don’t worry, let me write one for you.”

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