WASHINGTON (AP) -- The Food and Drug Administration says the industrial chemical melamine and a byproduct cyanuric acid have now been detected in four of 89 containers of infant formula made in the United States, doubling previously reported positive results. The contamination is extremely minute, at levels federal regulators say are safe for babies.

In November, The Associated Press reported previously undisclosed FDA tests, obtained under the Freedom of Information Act, showing that out of 77 containers of domestic infant formula tested, a can of milk-based liquid Nestle Good Start Supreme Infant Formula with Iron contained traces of melamine while Mead Johnson's Enfamil LIPIL with Iron had traces of cyanuric acid.

The FDA has now updated its response to the AP's FOIA request by posting results of 89 tests on its Web site. Those results show that two additional containers of Enfamil LIPIL with Iron had traces of cyanuric acid.

Separately, a third major formula maker -- Abbott Laboratories, whose brands include Similac -- told AP in November that in-house tests had detected trace levels of melamine in its infant formula.

Those levels were below what FDA found in the other formulas, an Abbott spokesman said, and below any national safety guidelines. FDA tested 37 different Abbott Laboratories formulas and had no detections.

Abbott Laboratories, Nestle and Mead Johnson make more than 90 percent of all infant formula produced in the United States. In addition to the Abbott formulas, the FDA tested five Nestle formulas, 21 Mead Johnson formulas and 26 products of a fourth company, PBM Products. Mead Johnson is a unit of Bristol-Myers Squibb Co.

Melamine at much higher levels was recently found to have contaminated milk products around the world and has been implicated in the sickening of nearly 300,000 babies in China and killing at least six infants there. Melamine is rich in nitrogen, which registers as protein on many routine tests. Authorities say the melamine was added to Chinese formula to artificially boost its protein levels.

The FDA and other experts said they believe the minute melamine contamination in U.S.-made formula had occurred during the manufacturing process, rather than intentionally. The U.S. government quietly began testing domestically produced infant formula in September, soon after problems with melamine-spiked formula surfaced in China. No Chinese manufacturers of infant formula have met requirements to sell their product here, according to the FDA.

Melamine can legally be used in some food packaging, and can rub off into food from there. It's also part of a cleaning solution used on some food processing equipment. Mead Johnson officials said the FDA had informed them of the test results and they were confident the levels of cyanuric acid are so low that they do not pose a health risk to infants. The company said it is considering changing the cleaning solutions it uses on its manufacturing equipment to reduce cyanuric acid contamination.

Though melamine is not believed harmful in tiny amounts, higher concentrations produce kidney stones, which can block the ducts that carry urine from the body, and in serious cases can cause kidney failure.

To date, here are the FDA results for detections in U.S.-made formula:

-- Two samples tested from one can of Mead Johnson's Infant Formula Powder, Enfamil LIPIL with Iron had cyanuric acid at levels of 0.412 and 0.31 parts per million;

-- Three samples tested from one can of Mead Johnson's Infant Formula Powder, Enfamil LIPIL with Iron had cyanuric acid at levels of 0.304, 0.406 and 0.248 parts per million;

-- Three samples tested from one can of Mead Johnson's Infant Formula Powder, Enfamil LIPIL with Iron had cyanuric acid at levels of 0.247, 0.245 and 0.249 parts per million;

-- Two samples from a can of Nestle's Good Start Supreme Infant Formula with Iron detected melamine at levels of 0.137 and 0.14 parts per million.

Before the contamination was disclosed, federal food regulators had said they were unable to set a safety threshold for melamine in infant formula. After the news reports, however, the agency set a threshold of 1 part per million of melamine in formula, provided a related chemical, including cyanuric acid, is not present. None of the formula has tested above that threshold.

Associated Press Writer Justin Pritchard contributed to this story.