The investigators are trying to get a handle on just how much pet food tainted with an industrial chemical called melamine made its way into products consumed by pets as well as by hogs and chickens.

On Monday, they announced that byproducts from tainted pet food had been used in chicken feed on some farms in Indiana. A few days earlier, they said that hog farms in seven states may have received tainted pet food for use as feed.

The pet food in question could be to blame for a wave of dog and cat deaths in March due to kidney failure. However, Dr. David Acheson, an assistant commissioner for the Food and Drug Administration, said the threat level to pets is greater than to livestock or humans.

"It was only a small portion of what the poultry was fed, and human consumers will only use poultry as a small portion of their diet," Acheson said.

By contrast, pets often eat the same product exclusively, he said.

"The dilution factors here are enormous," he said.

The pet food scare as well as earlier discoveries of E. coli in spinach and salmonella in peanut butter has led to concerns about the safety of the nation's food supply.

The FDA reacted to that concern Tuesday by naming Acheson as assistant commissioner for food protection. One of his first projects will be to develop a strategy that identifies potential gaps in the food safety system and what is necessary to address those gaps.

Meanwhile, Democratic lawmakers announced their own plans Tuesday for addressing food safety. Two lawmakers introduced legislation that would give the FDA the power to order mandatory recalls of adulterated food products, plus establish fines for companies that don't promptly report contaminated products.

"The evidence is clear our food safety system is collapsing and one of the main agencies charged with protecting it, is asleep," said Rep. Rosa DeLauro, the Connecticut Democrat and sponsor of the bill in the House. "This needs to change immediately. It is time to transform the FDA from the toothless agency it has become to one that takes the proactive steps necessary to protect our food supply and the public health."

Sen. Dick Durbin, D-Ill., is the lead sponsor of the bill in the Senate.

Lawmakers' concerns about the FDA also led to another hearing on Capitol Hill Tuesday as three former commissioners testified that the agency needs more money for inspectors and more regulatory authority over producers and suppliers.

"The fact is that food is a second-tier priority within the FDA," said Dr. David Kessler, who served as FDA commissioner in the administrations of the first President Bush and President Clinton.