State and federal investigators are using satellite photographs to find out who was responsible for an oil spill that threatens some of the richest bird habitat on the West Coast.

The spilled oil - an estimated 2,500 gallons, or about 60 barrels - formed a slick sheen about 10 miles long around a smaller ribbon of heavy dark tar balls. It was first spotted over the weekend about 10 miles from San Francisco.As crews began picking up tar balls Wednesday on beaches near Half Moon Bay south of San Francisco, wildlife officials feared that a change in winds could spell disaster for tens of thousands of birds. A few dozen have been killed or injured so far.