SAN JUAN, PUERTO RICO — The first warning sign came when a sharp-eyed worker sorting pills noticed that the odd blue flecks dotting the finished drug capsules matched the paint on the factory doors.

After the flecks were spotted again on the capsules, a blood-pressure medication called diltiazem, the plant began placing covers over drugs in carts in its manufacturing areas.

But the factory owner, Canadian drug maker Biovail Corp., never tried to find out whether past shipments of the drug were contaminated — or prevent future contamination, according to U.S. regulators.

Thirteen of the 20 best-selling drugs in the United States come from plants on this island. But an investigation by The Associated Press has found dozens of examples over four years of lapses in quality control in the Puerto Rican pharmaceutical industry, which churns out $35 billion of drugs each year, most of it for sale as part of the $300 billion market in the U.S.

An AP review of 100 pages of Food and Drug Administration reports shows even modern drug plants here under the watch of U.S. regulators have failed to keep laboratories sterile and have exported tainted pills.

"People would be shocked to find this whole variety of contamination," said Dr. Sidney Wolfe of the Washington watchdog group Public Citizen. "The common denominator of all these is there's really poor quality control."

FDA officials say the problems in Puerto Rico are proportionate with the large number of pharmaceutical plants here and generally no worse than those on the U.S. mainland.

Consumer advocates say they demonstrate the regulatory agency does not sufficiently monitor the industry across Puerto Rico and in the mainland.

The FDA issued a warning letter to Wyeth in May 2006, after consumers reported finding machinery pins inside bottles of Effexor, a leading depression treatment, and the acid reflux drug Protonix. The letter expressed concern that the plant was not "able to detect that the affected equipment was missing some of its parts." The Madison, N.J.-based company faulted mistakes by workers who packaged the drugs.

In another case cited in a June 2006 FDA inspection report, a plant owned by Teva Pharmaceutical Industries exported drugs — including the diabetes treatment metformin — even though they were known to contain small amounts of metal particles. The company had also received at least six consumer complaints of dark residue inside bottles or foreign material embedded in tablets, according to the report.

Teva's quality-control unit said the presence of some metallic material was to be expected because the manufacturing equipment is made of metal, according to the report.

Teva recalled 21 different drugs because of the inspection, according to FDA officials, and the Israeli drugmaker announced two months later it was closing the plant, citing a restructuring.