The FBI is investigating more than 30 people in the slaying of Capitol Police Officer Brian Sicknick during last week’s riot in Washington, DC, The Post has learned.

The wide-ranging nature of the ongoing probe was revealed by a law enforcement source briefed on the matter.

The disclosure followed Thursday’s arrest of Robert Sanford, a retired Pennsylvania firefighter who was allegedly caught on camera tossing a fire extinguisher off a balcony on the west side of the Capitol amid the Jan. 6 chaos.

The device struck the heads of three Capitol cops, only two of whom were wearing helmets, according to authorities.

Sicknick died a day after being hit in the head with a fire extinguisher, but the incident in which Sanford was charged is not connected to Sicknick’s slaying, law enforcement sources have told the Associated Press.