BEIJING, China (CNN) -- Chinese officials in the northwestern Xinjiang Uygur Autonomous Region reported the death of a fourth baby Thursday in the country's expanding contaminated infant formula case. The tainted milk powder has already sickened more than 6,200 babies.



Earlier in the day, authorities announced the arrest of 12 more suspects in the investigation, provincial police said.



The arrests bring to 18 the number of people detained, according to the Hebei Public Security Bureau.



Twelve of those arrested are involved in the collection of milk and the production of milk products, authorities said. Authorities accuse the other six of illegally selling the chemical added to the milk.



More than 1,300 infants are hospitalized, said Li Changjiang, China's director of quarantine and inspection, on Wednesday. Their conditions include malnutrition, kidney stones and acute renal failure.



Inspectors have been deployed across the country to test producers' inventories.



Of China's 175 baby milk powder production companies, 66 have stopped production, Li said. Watch parents scramble for safe milk »



Two brothers who sold fresh milk used to produce contaminated baby milk powder were arrested by Chinese investigators Monday. They could face death if convicted, according to China Daily, a state-run newspaper.



The raw milk had been watered down and a chemical added to fool quality checks, the newspaper said.



Investigators said the brothers confessed to watering down the raw milk and mixing in tripolycyanamide, also known as melamine. They said they did it to recover losses suffered when the factory rejected earlier milk shipments, the paper reported.  Learn more about the chemical melamine »



The brothers are charged with producing and selling toxic and hazardous food, which carries a possible death penalty, the paper said.



Health experts say ingesting melamine can lead to kidney stones, urinary tract ulcers, and eye and skin irritation.



The chemical is commonly used in coatings and laminates, wood adhesives, fabric coatings, ceiling tiles and flame retardants.  Watch CNN visit the company at the center of the scandal »



Thousands of tons of the tainted milk powder have been recalled, including pullbacks by Mengniu Dairy Group, China's largest milk producer, and the Sanlu Group.



Chinese investigators have found melamine in nearly 70 milk products from more than 20 companies, Li said Wednesday. Products made by Sanlu had the highest concentration of the chemical.



It is not the first time Sanlu has been connected to a scandal involving tainted milk powder, according to China Daily.



In 2004, at least 13 infants in the eastern Anhui province died of malnutrition after drinking milk powder that had little to no nutrition. The illegally manufactured milk was falsely labeled with the Sanlu brand, according to the paper.



The vast majority of the tainted formula was consumed domestically, but it is known to have been exported to Taiwan, Bangladesh, Myanmar, Yemen, Chad and Burundi, according to Li.



The U.S. Food and Drug Administration said no Chinese baby formula has been allowed on the market in the United States. In a statement on its Web site, the FDA said it had reached out to all five companies making formula in the United States and none has used formula or source materials from China.



This episode marks the latest in a string of tainted products produced in China:



CNN's Yuli Yang contributed to this story.
Who recalled batches of formula? What is China's biggest milk producer? How many babies have died? How many infants are known to be ill? How many infants are ill? What caused the babies' deaths? What were 12 people arrested for? How many arrests? How mnay companies stopped production? How many babies died?
Mengniu Dairy Group, Mengniu Dairy Group, 13 more than 6,200 more than 6,200 tainted milk powder contaminated infant formula case. 18 66 13
question: Who recalled batches of formula?, answer: Mengniu Dairy Group, | question: What is China's biggest milk producer?, answer: Mengniu Dairy Group, | question: How many babies have died?, answer: 13 | question: How many infants are known to be ill?, answer: more than 6,200 | question: How many infants are ill?, answer: more than 6,200 | question: What caused the babies' deaths?, answer: tainted milk powder | question: What were 12 people arrested for?, answer: contaminated infant formula case. | question: How many arrests?, answer: 18 | question: How mnay companies stopped production?, answer: 66 | question: How many babies died?, answer: 13