WASHINGTON — U.S. inspectors found some mostly procedural problems at a Chinese factory that supplied the main ingredient for the recalled blood thinner heparin _ but said Thursday they can't yet tell what is to blame for serious side effects.

"We're still considering all possibilities," said Food and Drug Administration compliance chief Deborah Autor. "We have not ruled anything out at this point."

Baxter International halted production earlier this month after the blood thinner was linked to four deaths and hundreds of reports of allergic-type reactions, including vomiting, nausea and difficulty breathing.

The FDA has completed an inspection of the Chinese factory, Changzhou SPL, that supplied the active ingredient, and took the unusual step Thursday of posting its draft inspection findings on the agency's Web site. The FDA is under intense scrutiny in the probe, after it acknowledged that an agency mix-up meant the Chinese factory had never been inspected before the heparin began selling.

Inspectors compiled a list of possible problems in how workers ensure they're producing quality drug.

Among the findings:

_ A lack of specific procedures outlining removal of impurities.

_ Workers reprocessed the drug that didn't meet specifications without determining what caused the initial problem.

_ No records showing suppliers' source of the products.

_ Some apparently improperly cleaned and maintained equipment.

Heparin, used to prevent blood clotting, is generally made from pig intestines. In China and other developing countries, tracing the source of animals used to make it can be difficult.

The FDA stressed that inspectors' initial findings are preliminary and must be further investigated. China isn't the probe's only focus; the FDA also is conducting inspections at facilities in the U.S.

The FDA said Thursday that it now has a total of 448 reports of possible problems in patients taking heparin from any manufacturer, but the agency thinks that those associated with the problematic Baxter product number 215.

Baxter hadn't formally recalled all of the problematic heparin vials because the FDA feared doing so would trigger a shortage. Thursday, Baxter took that next step, after the FDA verified that a second U.S. supplier has ramped up to fill the gap.

Of most concern are vials of heparin used in fairly large doses, such as for heart surgery or during dialysis. But also recalled Thursday, in what FDA said was a purely precautionary move, was Hep-Lock, a very diluted product used to prevent blood clots from forming in intravenous medication lines, because it was made with the same ingredient.

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On the Net:

FDA's Heparin update: http://www.fda.gov/cder/drug/infopage/heparin/default.htm