WASHINGTON - Federal officials still can't give the all clear when it comes to the nation's pet food supply, though they assured lawmakers they're aggressively checking stores and suppliers.

The Food and Drug Administration advised pet owners Thursday that recalled pet food may still be on the shelves in some retail establishments. 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. The FDA 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.

Sen. Dick Durbin, D-Ill., 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.

Elizabeth Hodgkins, a veterinarian, said pet food labels should not be able to make safety claims without rigorous ingredient testing by the manufacturer or the company that supplies the manufacturer. She said such testing does not occur.

“The pet food safety crisis is not an unfortunate aberration but part of mounting evidence of a systemic breakdown,” Hodgkins said.

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.

Ekedahl said the pet food industry was also forming a commission made up of industry and government officials to investigate how the pet food became tainted and to recommend steps that can be taken to improve safety.

“If you take one thing away from my remarks today, please understand this,” Ekedahl said in written testimony. “The answer to this problem is not additional regulation, rather it is enhanced communication.”

FDA officials did acknowledge that their communication with the public about the extent of the recall has not been clear at times. Sen. Herb Kohl, D-Wis., called the agency's Web site a bewilderment.

“We recognize there was a lot of confusion,” Sundlof said. “We didn't have all the answers ourselves.”