The federal investigation into the contamination of more than 1 million pounds of ground beef focused Monday on whether dangerous bacteria came from a slaughterhouse or perhaps from an unwitting employee of a Nebraska packing plant.

Agriculture Department inspectors were intensifying their testing for the E. coli bacteria at the Hudson Foods Co. plant in Columbus, Neb., officials said. The once-a-day sampling began Monday and will continue for 15 days.

The company halted shipments from the Columbus plant pending results of the tests, said Kenneth May, a director and scientific adviser for Hudson. That involves about 250,000 pounds of meat each day.

So far, government officials say there doesn’t appear to be any reason for additional recalls.