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WASHINGTON — A third round of testing will help to determine whether an envelope addressed to the White House does indeed contain cyanide.

Initial biological testing came back negative, the Secret Service said, but another test conducted Tuesday returned a “presumptive positive” for cyanide.

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

The Secret Service, which is responsible for the safety and security of President Barack Obama and his immediate family, said its investigation into the letter was continuing and it will have no additional comment on the matter.

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.