FBI
The FBI was in the first two seasons represented mainly by FBI Agent Nathan Green and also Gavin Orsay (aka. HEROnymous Bot) who works for the FBI in exchange for a "jail-free" life.
The FBI was hired by Frank Underwood's Chief of Staff, Doug Stamper, for helping Lucas Goodwin to start a cyber-terrorism act, because he (and Zoe and Janine) know too much informations about Peter Russo's death and Rachel Posner. The FBI also contacted Janine Skorsky.