Two suspected bombs have been intercepted one addressed to Barack Obama, the other to Hillary Clinton.
The US Secret Service says they were identified during routine mail screening and there was no risk to the former politicians.
The White House says it condemns the acts, calling them "terrorising" and "despicable".
One of them was discovered about 50 kilometres north of New York city, the other in Washington DC.
Investigators think it's linked to another device discovered on Monday at the compound of billionaire philanthropist, George Soros.