The FBI, U.S. Capitol Police and U.S. Postal Inspection Service are investigating letters containing suspicious substances that were mailed to “various addresses nationwide, including multiple Congressional offices,” according to a notice that senior staff in Senate offices received.

The offices of several Democratic senators were targeted, including that of Senate Minority Leader Chuck Schumer, D-N.Y., NBC News confirmed. Two law enforcement sources briefed on the matter confirmed that “dozens of letters” were sent, but because law enforcement continues to intercept the letters, they declined to put an exact number on how many have been received or seized.