<p>WASHINGTON (CNN) -- Preliminary tests on a white, powdery substance found in the mailroom of Senate Majority Leader Bill Frist indicate the presence of the deadly substance ricin, a Homeland Security official said Monday.</p>
<p>A U.S. Capitol police spokeswoman said the department is investigating the matter but would not comment on the substance found, saying only that preliminary tests were positive for a "hazardous substance."</p>