The Food and Drug Administration advised pet owners Thursday that recalled pet food may still be on the shelves in some stores. The agency asked retailers across the country to be vigilant in removing all products associated with the pet food recall, which began on March 16. Stephen Sundlof, director of the agency's Center for Veterinary Medicine, told lawmakers during a hearing that thousands of government and private sector workers around the country have responded to the contamination. Yet, he told lawmakers, he could not rule out the discovery of more tainted food.

"We do believe we've got the vast, vast majority off the market," Sundlof told members of a Senate Appropriations subcommittee.

On March 16, Menu Foods recalled 60 million cans of dog and cat food after the deaths of 16 pets, mostly cats, that ate its products. FDA regulators said tests indicated the food was contaminated with an industrial chemical, melamine.

At least six pet food companies have recalled products made with imported Chinese wheat gluten tainted with the chemical. The recall involved about 1 percent of the U.S. pet food supply.

Democratic Sen. Dick Durbin said the contamination showed that pet food as well as human food is at risk because of significant gaps in the system of regulations and inspections that governs the food industry. In particular, he said, the latest contamination shows that too few pet food manufacturers are being inspected.

"It appears that there is a light federal presence in this area and instead we rely on a patchwork of state inspection systems and voluntary guidance," said Durbin, who requested the hearing.

Durbin asked government regulators about a report some of the contaminated wheat gluten made it into human food. The FDA did discover wheat gluten imported from a different Chinese source that coincidently bore a lot number similar to some of the tainted wheat gluten, said Steven Solomon, of the FDA's Office of Regulatory Affairs. The FDA asked an unidentified company to halt distribution of products made with the ingredient until testing revealed they were free of contamination, Solomon said.

"We did those tests very rapidly," Solomon said. "All those tests were negative. All the wheat gluten from other suppliers has all tested negative to date."

Menu Foods was asked to attend the hearing, but it requested that the Pet Food Institute, a trade association for the industry, appear instead. The institute's president, Duane Ekedahl, told the committee that pet food already is perhaps the most highly regulated product on store shelves. He noted that manufacturers are governed by the FDA and the Agriculture Department as well as authorities in all 50 states.

"Pet foods are safe," Ekedahl assured the committee.