Officials are particularly concerned about
the letter sent to the Senate majority
leader, Tom Daschle, Democrat of South
Dakota. Several top scientists said today
that the powdery anthrax found in that
letter was advanced and highly
dangerous, contradicting officials who
had suggested over the past week that
the substance was relatively primitive.

In particular, the scientists said that it had been altered to reduce its electrostatic charge, making the spores less likely to clump together and more likely to float in the air. One expert said that only the United States, the Soviet Union, and Iraq were known to have developed the necessary technique to do this.