WASHINGTON (AP) — The Secret Service will have a third round of testing done to determine whether a letter addressed to the White House contains cyanide.

The Secret Service says initial testing came back negative, but a second test conducted yesterday returned a "presumptive positive" for cyanide.

A facility located away from the White House and its surrounding buildings had received the letter Monday for routine screening.

Suspicious letters often are sent to some of the country's leading politicians, including the president. Some test positive for hazardous substances while others include threats of death or other physical harm.