Chicago police said a man with a knife was taken into custody at Union Station in one of two incidents.
None of the mail has been deemed hazardous so far but it has caused evacuations.
Capitol Police said they temporarily locked down the headquarters of the Republican National Convention after a suspicious package with two vials of blood was found.
Federal authorities are investigating suspicious letters sent to election offices in five states.
A suspicious package was found at the police station, prompting a large response from emergency crews.
A series of threatening letters containing a mysterious white powder is raising concerns that Republican lawmakers in Kansas are being targeted.