Luckily, the letter was tested a few times in order to confirm the substance was actually there.
At first, the letter tested negative for any poisonous agents. However, it did return as a 'presumptive positive' for Cyanide.
Cyanide is easily a deadly poison that could have caused a lot of damage in Washington had it not been discovered.
Although the Secret Service hasn't reported who the letter was addressed to in the White House, all proper precautions have been taken in order to prevent the letter from getting close to anyone.
This is not the first time a dangerous letter has been addressed to the White House. Now, all letters have to be very carefully screened before heading anywhere close to the President's hands.